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Glossary of photographic and technical terms

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ABS: 'acrylonitrile-butadiene-styrene co-polymer' - a cross between plastic and rubber with excellent engineering properties, used to make underwater housings, cabinets for electronic equipment, solvent-weld plumbing fittings, etc.. ABS is attacked by many industrial cleaning agents, especially chlorinated solvents, ketones (e.g., MEK), and furans (e.g., THF). Cleaning with agents other than mild detergent and water is not recommended.

Deviation, Wandering away, The non-convergence of rays of light to one focus.

AC: Alternating current. An electrical current which changes over time, usually in a sinusoidal fashion. Usually associated with mains electricity.
     When Thomas Edison an his co-workers developed electrical lighting into a commercially practical technology, the original power distribution system used direct current (DC). The problem with DC is that it cannot easily be transformed from one voltage to another, and so the generator has to be operated at something close to the voltage which will be made available to the end user, i.e., a lowish voltage with a correspondingly high current for a given amount of transmitted power. Low voltage distribution systems unfortunately suffer from the problem of resistance, or voltage drop, in the transmission cables, and it was said that if a skyscraper had been supplied with DC, the lights at the bottom of the building would have glowed a lot brighter than the lights at the top. The problem was solved by the adoption of a low-frequency AC system (largely attributable to Nicola Tesla), ie., an electrical current which changes from positive to negative and back again, sinusoidally, at a speed too low to cause significant radiation loss but fast enough that no-one can see the lights flickering. The beauty of AC power is that it can be transmitted as high voltage, low current, and then changed to low voltage, high current for the end user by means of a simple magnetic device called a transformer (ie., two coils of wire on a common iron core). In America, the finally adopted AC power-line frequency is 60 cycles per second (60Hz). In Europe, the power-line frequency is 50Hz. The process of changing AC back into DC, for equipment which can only run on DC (such as electronic circuitry and early 20th Century subway trains) is called "rectification" (ie., 'putting-right').

a lens corrected for chromatic aberration (colour fringing) at two wavelengths in the visible spectrum. Most modern camera lenses are achromats.

Achromatic doublet: a lens in which achromatic correction is achieved by cementing together two lenses made from different types of glass.

Poly methyl methacrylate. A clear plastic with good optical properties and very high transparency. Refractive index: nd = 1.492.  Trade names: Perspex, Plexiglass, Lucite. Acrylic is attacked by chlorinated organic solvents. Stressed (i.e., shaped or moulded) acrylic will shatter in contact with alcohols, so be very careful about what you put on it if you want to clean it (mild detergent and water is preferred). Wiping with 60-80 petrol (cigarette lighter fluid) can be used to remove severe non-water-soluble adhesions.  

AE: Automatic Exposure. A method of linking the camera exposure settings to the ambient or reflected light level.

Automatic-Focusing. There are two basic types of automatic focusing system; range-finding systems, and contrast or image-sharpness sensing systems. Range-finding systems send out a signal; usually an infrared beam (but sometimes ultrasonic), and see how long it takes for the reflected signal from the subject to come back. Such systems can be fooled by windows, and they don't work underwater because the water absorbs or modifies the signal. Contrast sensing systems determine image sharpness in much the same way as the human eye, and are much more difficult to fool, but they can still be fooled by repeating patterns, and they don't work if the subject lacks detail. Range-finding systems are sometimes called 'active autofocus' because they send out a signal, whereas contrast sensitive systems are called 'passive autofocus' because they don't. This is one case where it is usually better to be passive.

Ah, Ampere hours: The capacity rating of an electrical cell or battery. One point to watch when interpreting ampere-hour ratings is that the capacity of a battery or cell varies depending on the rate of discharge. The figure is normally specified at the 10 hour rate. E.g., A 10Ah battery will deliver a current of 1 amp for 10 hours. It will however, deliver a current of 10 amps for somewhat less than 1 hour. I.e., the effective capacity is reduced at high discharge rates (and also, not greatly increased at very low discharge rates). In combination with the average battery terminal voltage throughout the discharge cycle, the battery capacity is also a measure of the available stored energy. E.g., a 6V 10Ah battery can deliver 6 Watts (6 Volts x 1 Amp) for 10 hours, i.e., 6 Watts for 36000 seconds = 216000 Joules (216KJ). That is a lot of energy. If you short-circuit a battery, nearly all of the stored energy will be dissipated in the battery itself, which should explain why batteries can pose a severe fire hazard if misused or mishandled. Some modern batteries have energy densities close to that of Dynamite.

Al, Aluminium, Aluminum: Humphrey Davy, the discoverer of this chemical element, originally named it "A l u m i u m". He then changed his mind and called it "A l u m i n u m", and this naming convention is the one adopted in North America. The British Chemical Society however, felt that element names should always end with "-ium", and so changed the name to "A l u m i n i u m". The Americans have the wishes of the discoverer on their side.

Amp, Ampere, A: The unit of electrical current, ie., quantity of electricity per unit time. Current flows through wires and conductors, whereas voltage (electrical pressure-difference) appears across things like generators, batteries and resistors.

Angle of Coverage (of a lens): The angle between rays of light entering a lens from the extreme corners of a scene.

Angle of Coverage (of a flash unit): The angle of the light-cone or pyramid emanating from the light source. The quoted angle of coverage for a flash unit corresponds to the angle between two rays, on opposite sides of the beam axis, chosen so that the light intensity is half of that at the beam centre. I.e., it is assumed that the useful field of illumination ends at the point where the light intensity has fallen by 1 EV (1 stop) relative to the centre. Circular light sources are given a single value for angle of coverage, rectangular light sources are given two values (Horizontal and Vertical).

Anodisation (Anodization): An electrolytic process for forming a stable film of aluminium (aluminum) oxide on the surface of aluminium alloys. Aluminium will normally grow an oxide layer on contact with air; but the layer formed in the anodisation process is harder and less porous, giving improved corrosion and abrasion resistance, and can be made to incorporate pigments as an alternative to painting the finished product.

Anti-reflection coating: Reflections which occur at optical surfaces are a general source of lens flare (i.e., image artifacts due to bright light sources outside of the field-of-view).  Reflection is reduced by reducing the abruptness of the change in refractive index at an optical surface.  This can be achieved by coating the surface with a very thin layer of some material which has a refractive index intermediate between the refractive indeces of the two media.

Aperture: The variable diameter hole used to control the amount of light passing through a lens. See f-stop.

Apochromat: a lens corrected for chromatic aberration (colour fringing) at three wavelengths in the visible spectrum. Traditional apochromatic lenses using spherically-ground glass elements are complicated and expensive. Apochromatic lenses can also be produced by using aspherical moulded lens elements, but moulding is generally an inferior process to grinding, and so, while a lens with moulded elements may be strictly described as apochromatic, that doesn't always mean that it will be superior to a traditional achromat (and if the elements are pressed out of plastic it may well be awful - caveat emptor). This is not to say that modern moulding methods cannot produce results equal to grinding, they can, but high quality optical mouldings are not to be expected in cheap lenses.

APS: Advanced Photographic System.  Photographic film with integral magnetic recording strip for data.  25.1 × 16.7mm image format.
APS-C:  Term used for camera sensors of format dimensions similar to those of APS.

AR: Anti-reflection (coating)

B, Bulb sync.: In old-fashioned flash-bulb photography, the bulb must be ignited a few milliseconds before the shutter is opened, to give it a chance to get going. Alternatively, the shutter must be opened for a relatively long time, to make sure that the bulb gives out its light while the shutter is open. Nowadays, the camera 'B' position is used for time exposures, it being a setting where the shutter will remain open for as long as the shutter button is held pressed.

BA: Screw-thread system for small instruments proposed in the 19th century by the British Association for the Advancement of Science. BA screws are based on metric dimensions with a constant pitch gradient. The pitch angle is 47.5°. Some BA sizes will fit with modern metric parts, e.g. 0BA fits M6×1, 13BA fits M1×0.25. Now largely obsolete.

Backscatter: Light reflected from suspended particles in the water, a problem in underwater flash photography caused by having the flash too close to the camera lens. It can manifest itself as anything from a slight fog, to a haze of bright specks obscuring the picture. In extreme cases, particles close to the camera shine so brightly that they cause lens flare, in which case some of the specks will appear to have the same shape as the lens iris. The problem is minimised by using an external flash, either mounted on a long adjustable arm, or un-mounted and positioned by hand.

Unit of pressure in the metric system. Standard atmospheric pressure is 1.01325 bar. 1 bar = 100 000 Newtons / metre squared (100 Kilo Pascals).  As you dive underwater, due to the weight of the water above you; the pressure increases by 1 bar for every 9.806 metres of depth.  To a good approximation, we say that the pressure increases by 1 bar for every 10m underwater.

Bayer mosaic: The pattern of red green and blue filters used to separate colour in a conventional CCD or CMOS camera sensor. US Patent No. 3971065, 1976.
See pixels and resolution article for more information. 

bits-per-pixel (bpp): A measure of tonal rendering capability in digital pictures. A good impression of continuous tonal rendering can be created by having 256 possible levels of brightness for each of the colours red, green, and blue. 256 levels can be represented by an 8-bit (1 byte) binary number, so it takes 3 x 8 = 24 bits to create the impression of continuous tonal rendering. 24 bits corresponds to about 16.7 million possible unique values of hue and brightness. It is better to start with considerably more than 8 bits per colour if any image adjustment is to be carried out, so that there will be enough information left to give 8 bits per colour in the final result.

BK7: A widely used high quality optical crown glass, relatively hard with good scratch resistance.
nd = 1.5168 @ 587.6nm.  More information.

BNC connector: Bayonet Neill-Concelman (named after the designers, Paul Neill and Carl Concelman). Coaxial connector used for video, ethernet, and radio frequency signals.
[Rad Com (RSGB publ), October 1995, p67. QEX (ARRL publ), May 1985, p2.]

Buccaneer connector: Multi-pole (multi-pin) waterproof connector, manufactured by Bulgin, with an ingress protection rating of IP68 1 bar (ie., submersible to a depth of at least 10m, and in practice, a lot more).

Burn-time: The time for which a lamp will operate after starting with a fully charged battery.

Bulkhead: A wall or partition in maritime parlance.
Bulkhead connector: An electrical connector which mounts on a panel or bulkhead.

Buoyancy: The propensity of an object to float (positive buoyancy) or sink (negative buoyancy) or to do neither (neutral buoyancy).

Camcorder: Sony always called its tape recorders 'Tapecorders', thereby avoiding a pronunciation difficulty for native Japanese speakers. A combined camera and video recorder therefore naturally became a 'Camcorder'.

Camera: Latin and Italian: a small room or bedroom. English equivalent: Chamber. The word Camera came into the English language as a shortened form of the phrase 'camera obscura', a darkened room. The invention of the camera arises from the observation that, if a house is completely shuttered, light coming through a chink or keyhole will project an upside-down laterally-reversed image of what lies outside on to the opposite wall. Artists developed the secret practice of setting-up such a room (or tent, or box) and placing an easel inside, so that they could sketch over the resulting image and use it as the basis for a painting. A study by the artist David Hockney (Secret Knowledge: Rediscovering the lost techniques of the Old Masters, ISBN 0-500-23785-9) argues that the camera obscura, in combination with various mirrors, lenses, and tracing screens, was used by artists from about 1430 onwards - and that the works of many of the Old Masters are effectively photomontages (the extraordinary skill of the artist being that of rendering the optical image in paint). A subsequent study of the use of perspective in Hockney's principle example (Arnolfini & his Wife by Van Eyck) has argued that Van Eyck did not use optical methods in setting out this work (see Scientific American, Dec 2005, p52-59), although an alternative interpretation is that the artist varied his optical setup depending on which part of the picture he was rendering. The word 'photography' means 'drawing with light', ie., dispensing with the artist, and using light-sensitive chemicals instead. Because 'camera' means 'bedroom', Italians use the terms 'fotocamera' and 'maccina fotografica'.

CCTV: Closed-circuit Television, i.e., TV not involving radio transmission or long distances. A somewhat old-fashioned term which confuses the difference between video and TV. Television means vision at long-distance, usually involving transmission by radio, radio carrier in long-range cables, and fibre-optics. The term 'CCTV' is therefore tautologous (short-range long-range vision), and a CCTV camera is nowadays more sensibly referred to as a 'video camera' or a 'surveillance camera'.

Capacitor: An electrical storage device. In a flash unit, energy is stored in a high-voltage capacitor for rapid discharge into the flash tube (See also inverter).

Caveat: Beware. of which to beware.
Caveat Emptor: Buyer beware!

CCD: Charge Coupled Device. An integrated circuit (micro-chip) consisting of a group of charge storage cells (tiny capacitors) with the ability to pass charge from one to the next, in a line, like a bucket-brigade (ie, like a group of fire-fighters passing buckets from one to the next). Originally conceived in the mid 1960s as an analogue delay line for processing Radar images, someone came up with the idea of exploiting the fact that the cells are sensitive to light, and setting them out in an array, like the lines of a TV picture. Thus was born the replacement for the fussy and troublesome TV camera tube. Most modern video cameras use a single CCD array with a colour mosaic filter to separate red, green and blue. Early professional cameras split the light into red, green, and blue optically, and used 3 CCD arrays, but as the number of pixels increased (for HD and stills capture), physical alignment of the three pictures became impossible, and single-CCD (mosaic filter) solutions had to be found.

Cell (electrical): An electrochemical energy source. A device with a particular voltage difference across its terminals. Electrochemical cells are often connected in series to form a 'battery of cells', commonly known as a 'battery'. Hence, for example, a 12V battery may be constructed by connecting ten 1.2V cells in series. A cell is composed of two half-cells, each comprisong an electrode (often an electrically conducting rod, plate, film or cannnister) and an electrolyte (a chemical solution). The two half-cells may use the same electrolyte, in which case the two electrodes are immersed in the same solution; or they may used different electrolytes, in which case the two electrolytes are separated by a semi-permeable membrane. The common Leclanché cell (also known as the zinc-manganese cell), has a positive electrode consisting of a porous container filled with manganese dioxide powder with a carbon rod inserted into it for connection, a zinc can for the negative electrode, and a solution of ammonium chloride for the electrolyte.

Cell, Corrosion: When two dissimilar metals in contact are immersed in water containing dissolved salts (an electrolyte), a short-circuited electrochemical cell is created. The action of the cell is to etch or consume one of the metals thereby producing electrical energy which is immediately wasted as heat. One way to stop the corrosion is to reverse-bias the corrosion cell, i.e., apply a voltage in opposition to the voltage generated so that no current flows. This is done by creating an additional corrosion cell using a metal which is more readily consumed than the metal to be protected. Aluminium, for example, can be protected by attaching a block of magnesium to it, this attachment being called a 'sacrificial anode'.

C-Mount: Screw-in lens mount used on some ciné and video cameras. 1" diameter thread with 1mm pitch.

CMY: Cyan, Magenta, Yellow. The 'subtractive' primary colours. Colour-space representation used in conventional print photography.

CMYK: Cyan, Magenta, Yellow, Black. The colour-space representation used by ink-jet and other printers. The letter 'K' is used for black because 'B' already stands for blue. The extra black ink is used to overcome the density limitations of the other inks, i.e., C+M+Y usually comes out dark brown rather than pure black. Also, it is a waste of money to print black and white documents and text using exotic coloured inks.

Colour, Color: Colour is what passes for spectroscopic analysis in the rather limited human (i.e., primate) visual system. A pure monochromatic light source will produce an impression of colour which depends on the wavelength of the light falling on the retina. The eye and the brain work together to analyse the input according to the relative levels of stimulation received by three types of cone cell, which have sensitivity peaks at wavelengths of 560nm (red), 530nm (green), and 424nm (blue) (nm = nano metres). The crudeness of this analysis makes colour photography (as we know it) possible, because we can fool the eye into seeing almost any colour simply by superimposing three light sources (R, G and B) and adjusting their relative intensities. Photographic (synthesised) colour is an illusion specifically tailored to (and exploiting the limitations of) the primate visual system; so don't expect non-primate animal species (even other mammals) to make sense of photographs and TV pictures. For more information on the evolution of colour vision in vertebrates and the differences between species, see "What Birds See", by Timothy H Goldsmith, Scientific American July 2006. p50 - 57.

Colour space: The set of colours which can be represented by a particular imaging system. There is considerable latitude of choice in selecting the exact wavelengths used to stimulate the red, green, and blue receptors (cone cells) in the eye. Different choices, and imperfections in the spectral purity of the light sources or dyes used, affect the colour representation capability (colour space) of the system. Synthetic (RGB or CMY) colour systems moreover, cannot properly recreate colours which correspond to the violet (450 - 390nm) part of the spectrum (magenta = red + blue is the best we can do), which is why photographs of bluebells always come out wrong. When editing images, it is best to use a working colour space which corresponds reasonably closely to that of the intended display device.

Colour Temperature: The temperature of an equivalent hot black body. A theoretical black body is a perfect absorber of light when cold, and hence a perfect emitter of light when hot (i.e., when glowing). The black body is equivalent to the light source in question in the sense that it produces the same relative amounts of red, green, and blue light. Colour temperature is measured in Kelvin, K (degrees above absolute zero).
    If an object is heated in a furnace, or by passing an electric current through it, it glows with a particular colour (red, orange, yellow, white, or bluish white) depending on the temperature. If the emitted light is free from spikes or gaps in its spectrum at particular wavelengths (ie., colours), then it is known as 'black body radiation'. Tungsten light-bulbs produce good black-body radiation. Electronic and gas-discharge light sources (e.g., flash tubes) do not produce perfect black-body radiation, but if there is enough red, green and blue light in the mixture, the source can often be correlated to a black body at a particular temperature. The correct term to express the colour rendering quality of a particular light source is therefore "correlated colour temperature". Standard photographic daylight is assumed to have a colour temperature of about 5500K. Xenon flash has a correlated colour temperature of about 6000K, which is fine for most purposes, but sometimes a little too blue. Underwater flash makers sometimes use a yellow reflector or filter, to reduce the flash colour temperature to around 4500K, to compensate for the blueness of the water. Domestic tungsten light-bulbs have a colour temperature of about 2950K, which is very orange. Quartz-halogen lamps have a colour temperature of 3200 - 3400K, which is yellowish, but electronic cameras can compensate for this by adjusting the relative sensitivities of the red, green, and blue channels (white balance). Humans do not usually notice the effective temperature of a light source except by comparison, because our eyes adjust automatically (look at the colour of a tungsten light-bulb while there is sunlight streaming through the windows - the electric light seems orange or yellow in the daytime, but perfectly white at night).

Complementary Colour: The colour of light which, when added to a given colour in the correct proportion, produces white (or grey, i.e., an illumination without a colour cast).
Red + Cyan gives white. Green + Magenta gives white. Blue + Yellow gives white. Hence Red and Cyan are a pair of complementary colours, so are green and magenta, so are blue and yellow.

Compression (of data): The removal of redundant or repetitive information from a data set (e.g., a file) in such a way that the original data can be reconstructed.
Lossless compression: compression which can be reversed without loss of information (e.g., LZW).
Lossy compression: compression which does not permit exact reconstruction of the original data (e.g., JPEG).

Concave lens: A lens in which the principal optical surface curves into the body of the glass, ie., the glass has a a hollow or dent.

Convex lens: A lens in which the principal optical surface bulges outwards.

CRT: Cathode Ray Tube. Term used for the large glass vacuum tube used in television or video displays and oscilloscopes (but slowly being superceded). The term was coined in the 19th century, before anyone knew that the mysterious 'cathode rays' which would cause a zinc sulphide screen to glow, were actually the sub-atomic particles we now call electrons. It became impossible to change the archaic name, because the Americans adopted the term 'electron tube' to refer to the device which the British scientist Sir Ambrose Fleming preferred to call a 'valve' (because it only conducts electricity in one direction). Fleming's valve was one of Mr Edison's light bulbs with an extra plate inside it to capture the electrons emitted by the hot filament. A CRT is a type of electronic valve (or electron tube) where the electrons are focused into a beam and fired at a phosphorescent screen. In a video display, magnetic fields produced by coils of wire fitted to the neck of the tube steer the electron beam rapidly so that it can be used to trace a series of horizontal lines on the screen (a raster).

CU: Close-up.

CVBS: 'Chrominance, Video, Blanking, and Synchronisation', ie., a composite video signal.

D8: Digital 8: Digital video recording system using 8mm tape cassette.

DC: Direct current (as opposed to alternating current); an electrical current which does not alternate or change significantly with time (at least over a moderate interval), such as might flow when a light bulb is connected to a battery. Never say "DC current" (direct current current), just say "DC".

DC-DC Converter: An electronic device which converts from one constant voltage to another (usually higher) constant voltage, e.g., 6V in, 330V out, as used in typical small flash units. A DC-DC voltage up-converter may also be called an inverter.

Delrin: Acetal Homopolymer. Engineering plastic with sufficient hardness and dimensional stability for the manufacture of precision components (e.g., screw-threads).

Depth of field: No lens can produce a perfectly sharp image. This means that there will be a range of focus settings for the lens which will give no significant improvement over an exact setting. Conversely, for a given distance setting, there will be a range of lens to subject distances over which focusing will be adequate or not-improvable. This range is called the depth of field. The depth of field of a lens improves as the aperture of the lens is made smaller, although maximum depth of field is not the same as maximum optical resolution. The trick, in photography, is to choose the aperture so that the depth of field embraces the whole depth of the subject from the furthest to the nearest point. Good lenses are provided with depth of field markings to assist in this matter, and good SLR cameras have a 'depth-of-field preview' button, which closes the lens down to the aperture which will be used when the picture is taken.

Diffuser: A device which increases the effective area of a light-source, i.e., makes it less point-like. A piece of opal or translucent plastic which can be fitted to the front of a light source. A diffuser helps to even-out the distribution of light intensity in the field of illumination, and reduces the problem of specular reflection from the subject.

Digital zoom: In-camera image cropping.  A feature beloved of marketing departments for the purpose of making the camera zoom-range specification seem better than it actiually is.

Diopter:: A unit used to express the power of magnifying glasses. Screw-in magnifying lenses, which can be fitted to the front of a camera lens, are sometimes referred to loosely as 'diopters'.
     The power of a lens in diopters is the reciprocal of its focal length in metres, 
Macro Lens Diopters Magnification
(Diopter / 4) +1
Maximum Focus Distance
1m / Diopter
  +0.5 1.125x 200cm 
  +1 1.25x 100cm
  +2 1.5x 50cm
INON UCL-330 +3 1.75x 33.33cm
Epoque DML-2 +4 2x 25cm
Subsea +5 +5 2.25x 20cm
  +6 2.5x 16.66cm
  +7 2.75x 14.29cm
INON UCL-165 +7.37 2.84x 13.57cm
  +8 3x 12.5cm
  +9 3.25x 11.11cm
Subsea +10
+10 3.5x 10cm
  +15 4.75x 6.66cm
  +20 6x 5cm
  +25 7.25x 4cm


The neat thing about diopters is that if you stack several lenses, the new magnifying power can be obtained simply by adding together the diopters of the individual lenses; i.e.,
D = D1 + D2 + . . . . . . . etc.
This formula holds good only if the lenses are close together, i.e., well inside each other's focal distance. Thus, if you stack a 1 diopter lens with a 2 diopter lens, you get a 3 diopter lens; but that doesn't mean it's a good idea. Stacking lenses like this is a useful practice while you're trying to determine the magnification power needed for a particular job; but if you want best corner to corner image sharpness, you should then obtain a single lens of the right power.
     Lenses which are overall convex (bulging) have positive diopters. Lenses which are overall concave (indented) have negative diopters. The low-cost close-up lenses designed to screw into the camera filter ring are usually convex on one side and concave on the other, but the convex part is more curved than the concave part, so overall, it's a magnifying glass. This concave-covex structure is called a `meniscus' lens, and is used because it gives the best minimisation of chromatic aberration for a lens constructed from a single piece of glass. Needless to say, adding such a lens to your camera causes a reduction in optical resolution. Far superior results can be had, at a price, by using two-element achromatic close-up lenses, such as those manufactured by Nikon and Canon.
diopters Meniscus lenses for screwing into the camera-lens filter thread. The lens holder has a male thread at the back and a female thread at the front, to allow stacking and the use of additional filters.

Dispersion: The separation of white light into its component colours. Dispersion in optical media (such as glass) occurs when the refractive index varies with wavelength. It is generally undesirable in lenses, since it leads to chromatic aberration.

DNR: Digital Noise Reduction. Clean-up process used when transferring analog video signals into the digital domain.

Dome port: Lens port constructed as part of a sphere of uniform thickness. The dome port corrects for the magnifying effect which occurs at an air-water boundary, and thus preserves the image geometry and angle of coverage. Contrary to popular myth, the dome port does not correct for all optical aberrations at the boundary, and actually introduces some. Introduced aberrations are reduced as the radius of the dome increases, but the problems of excessive buoyancy and delicacy of the outer optical surface impose practical limits on dome size.

Duo connector: Sea & Sea proprietary underwater connector used on sync. cables and flash units. It is called 'Duo' because the electrical specification supports both the Motormarine II and Nikonos V flash interface protocols.

DV: Digital Video

DX-coding: Pattern printed on the side of a 35mm film cassette which is used by the camera to sense the required ISO/ASA film-speed setting. The film speed is sensed by means of electrical contacts pressing against the canister, so dirt can cause mis-sensing. DX-coding is meant to simplify photography; but if no over-ride is provided, it can be a pain for advanced photographers who want to deviate from the film manufacturer's recommendations.

Electromagnetic Radiation:
Radio waves, infrared, visible light, ultraviolet, X-rays and gamma rays are all manifestations of the same thing: electromagnetic radiation. The difference between the various types is a consequence of the difference in frequency (or wavelength). The theory was worked out in the 19th century by James Clark-Maxwell; who showed that electricity and magnetism are related and that light consists of electric and magnetic fields linked together as they travel through space. Maxwell's work set the stage for the development of radio and countless other technologies which we nowadays take for granted. The relationship between frequency and wavelength is: c=fλ, Where c is the speed of light (299 792 458 metres per second), f is the frequency, and λ (the Greek letter lambda, lower case) is the wavelength, ie., the distance the wave travels as it undergoes one complete cycle. The middle of the medium-wave broadcast band is about 1MHz (one million cycles per second) and the corresponding wavelength is 300 metres. The wavelength at the middle of the visible spectrum is about 500nm (nanometres = thousand millionths of a metre), and the corresponding frequency is 6 × 1014 Hz, ie., 600 000 000 000 000 Hz (600THz or 600 Tera Hertz). Radio signals are normally characterised by their frequency, but light is normally characterised by its wavelength because the numbers for frequency are messy to write down without using scientific notation.
     Electromagnetic radiation is also sometimes split into two classes: ionising radiation, ie., radiation which has sufficient energy to knock electrons out of atoms and molecules, and non-ionising radiation, which hasn't. Radio and infrared are classed as non-ionising radiations (although some things can be ionised by infrared), whereas visible light, UV, X-rays and gamma rays are definitely ionising radiations. Our body senses are divided according to this distinction; in that we sense infrared through the skin by its heating effect, whereas we sense visible light through our eyes by its ability to split molecules of light-sensitive pigment. Insects can also see in the UV part of the spectrum, whereas to the human body, anything of shorter wavelength than visible light is simply harmful.

E/O connector: Electro-Oceanic: A type of wet-connector with two contacts, in appearance like a rubber jack-plug.
eo_m E/O Connector eo_f

E&OE: Errors and omissions excepted. Legally, this means that the information given cannot form part of a contract, i.e., it is not binding on the seller because it may require corrections.

EV: Exposure value. A change in exposure of 1EV corresponds to a halving or doubling of the intensity of light falling on a film or sensor, i.e., a 1EV change is equivalent to a change of 1 f-stop, or a halving or doubling of shutter speed or ISO film speed.

ewa-marine: Pronounced "ay-va marine".  German manufacturer of flexible underwater camera housings.  Such housings are known in the film and broadcast industries as "splashbags".

EVIL: Electronic Viewfinder, Interchangeable Lens.

EXIF: Exchangeable Image File. An extension to the JPEG file format to include information provided by a digital camera: time, date, firmware version, focal length, aperture, shutter speed, exposure value, etc.

Exposure: The product: light intensity × time.

Exposure bracketing: the practice of taking a series of photographs, usually at 1EV or 1/2EV increments on either side of the expected correct exposure, in order to obtain a range of images with slightly different exposures from which the best may be chosen. Exposure bracketing is of particular value when using a recording medium with limited exposure latitude, such as slide film, or digital cameras which only give 24bit output.

Extension tube: A lens focuses at infinity when the distance from the exit pupil of the lens to the film or sensor (the focal plane) is equal to the focal length. As the lens is moved away from the focal plane, closer objects come into focus, but the amount of extension permitted by the focus barrel (the thing you turn) is limited at some point by a decision made by the lens manufacturer. To make a lens focus closer than the designer originally intended, an extension tube can be fitted between the lens and the camera. Extension tubes for system (SLR and EVIL) cameras can have have control linkages or electrical connections passing through them to retain any automatic functionality of the lens. Manual extension tubes are simply tubes, with a male lens mount on one end and a female mount on the other. Different length tubes give different ultimate magnifications with a particular lens. Note that unless the lens is designed for the purpose, it will be taken out of its optimum focusing range when a tube or bellows extension is used and will therefore not give its maximum possible resolution.

Fasteners: Generic term or collective noun for nuts, bolts, screws, washers, and other parts or devices used in attaching one object to another.

FE: Fisheye

Fe: Iron (ferrum)

Field of View, FOV - of a lens: same as angle of coverage.

Filter: Any device which modifies a signal is a filter. Therefore, it is legitimate (but not good usage) to describe a close-up lens as a filter, the real reason for doing so being that close-up lenses screw into the camera filter-ring. In photography, the term 'filter' is normally reserved for a device (usually a sheet of glass or plastic) which attenuates (reduces) all or part of the light passing through it. A Neutral-Density (ND) filter causes an equal reduction across the whole visible spectrum and is used when the light is too bright for the camera equipment, or to modify the required iris setting as a way of controlling depth-of-field. Most other filters modify the colour or the light in some way, or remove troublesome invisible components. A UV filter removes ultraviolet light, to prevent haze, but leaves the visible spectrum virtually unchanged. An underwater colour-correction (UWCC) filter has an orange or magenta colour to compensate for the cyan-blue or green colour cast introduced by photographing through water; an there are a host of other colour-modifying filters for different applications.

Filter thread: Except for the products of awkward manufacturers, camera filter screw threads are international standard (ISO) metric (M) threads with a pitch distance of 0.75mm. If you measure the inside diameter of a female filter thread (using engineer's calipers), adding the pitch distance to the result gives the nominal thread diameter plus about a 10th of a mm. E.g., a camera filter thread measures 57.35mm. Adding the pitch distance gives 58.1mm. The filter thread size is fully specified as M58×0.75, but usually abbreviated to M58.

FireWire: See i.Link

Fisheye: Wide angle lenses can be designed in two ways: They can be corrected to make all straight lines appear straight (rectilinear correction); but this type of correction introduces perspective distortion, ie., assuming a flat object, it exacerbates the fact that points at the edge of the field are further away than points in the middle. The alternative is to avoid rectilinear correction, in which case perspective distortion is minimised; but rectangles appear to bulge in the shape of a barrel, and the only lines which remain straight are those which pass through the exact centre of the field of view. The latter type of lens is called a 'fisheye' lens.

fisheye distortion
Fish-eye distortion of rectangular test card
Fish-eye photograph with vanishing point
placed at the centre of the image

Flare: A major problem in lens design is that of minimising reflections from the surfaces of the lens elements. Such reflections result in multiple images, and in light from bright objects outside the field of view arriving at the film or sensor. The various optical effects which can occur due to internal lens reflections are known collectively as 'flare'. Reflections occur at boundaries where there is a sharp change of refractive index (e.g., on going from air into glass) - the sharper the boundary the stronger the reflection. Modern lenses have anti-reflection coatings, which make the change of refractive index more gradual and so reduce (but do not eliminate) flare. All compound lenses will exhibit some flare when shooting into the sun. Strong reflections from scene highlights will also cause visible flare. Some lenses can be fitted with an external 'anti-flare hood' which is designed to cut off all light-rays which do not originate from objects within the field of view.

Focal length, f: The distance at which a lens brings light from infinity to a point. The focal length of a simple symmetrical lens, is the distance between the image plane (the surface on which the image is formed) and the middle of the lens, when light from infinity is brought to a focus. You can estimate the focal length of a simple lens by projecting an image of the sun onto a heat-resistant surface, and measuring the distance from the middle of the lens to the image (do not look at the sun through a lens, do not focus an image of the sun onto your skin). Strictly, the place to measure from is the exit-pupil of the lens, which is where the iris (aperture) appears to be when you look into the back of the lens. Most camera lenses are not symmetrical however, i.e., the actual focal length is different if you turn the lens around, in which case, the figure quoted is the focal length of an equivalent symmetrical lens. Many camera lenses also are of the retrofocus type, i.e., they have additional optics at the rear to allow them to sit close to the film plane, and the simple method for estimating focal length given above will produce misleading results.
     Photographers often talk of focal length in the context of a given camera format (usually 36 × 24mm), in which case the term serves a shorthand for the associated angle of coverage. Note however that the commonly understood relationship between focal length and angle of coverage breaks down when you change the optical medium; i.e., in the absence of a correcting element (such as a dome port) lenses of a particular specified focal length are not as wide-angle underwater as they are in air.

Format: The 'format' of a film or camera is the dimensions of the image area (or frame). The 35mm (full-frame) film format used by stills cameras is 36 x 24mm. The 35mm (half-frame) format, used by motion-picture cameras is 24 x 18mm. The 2¼" square (medium) format is 57.15 x 57.15mm (not 60 x 60mm as is sometimes stated).
     Digital cameras use a wide variety of formats, making it extremely difficult to relate the focal lengths of the lenses used to the actual angles of coverage achieved. This problem is usually solved by giving a focal length multiplier, i.e., a number which when multiplied by the actual focal length, gives the equivalent focal length for the 35mm film format.  
     For a list of format sizes and focal length multipliers, and the implications of format choice with regard to image quality, see the Image formats article.

Four Thirds: An "open-standard" image format and common lens-mount specification for high-quality digital cameras, intended to make lenses from different camera manufacturers interchangeable (see It is supported by Olympus, Kodak, Fujifilm, Panasonic, Sanyo, Sigma, and others. The specification dictates a senor diagonal of 21.63mm (half that of the 35mm format, giving a focal length multiplier of 2). The original Four Thirds system was intended for SLR cameras. A variant "Micro Four Thirds" has a reduced lens to focal-plane distance specification and is designed for for mirrorless live-preview (EVIL) cameras. Four Thirds lenses can be used on Micro Four Thirds cameras by means of an adapter (an extension tube with electrical contacts).

FOV: Field of view. Usually synonymous with Angle of Coverage.

f-stop: The illuminating power of a lens is a function of the focal length of the lens and the diameter of the hole (aperture, iris) through which the light is allowed to pass. The amount of light falling on a film or electronic sensor can be adjusted by changing the diameter of the aperture; but instead of recording the actual diameter, we can treat all lenses equivalently if we record the aperture diameter as a function of the focal length. Thus we record apertures as f/n (focal length divided by a number n). eg., a 50mm lens at f/2 has an aperture of 25mm, an 80mm lens at f/2 has an aperture of 40mm, but both have the same illuminating power. A standard series of apertures has evolved such that the illumination doubles or halves with each click-stop (f-stop). If you double the diameter (or radius) of a circle, you quadruple the area (area = πr²), and it is the area which determines the level of illumination. Thus, to get from one f-stop to the next, the number n is not doubled, but multiplied by the square root of 2 ( √2 = 1.414). The standard f-stop series thus comes out as: f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, etc., ie, the number doubles for every 2 steps in the series. The smaller the number n, the larger the aperture. The aperture designation "f/22" incidentally is an ancient mistake. A more exactly calculated aperture value is f/22.6 which, rounded to the nearest whole number, should really be called "f/23".

GIF: 'Graphical Interchange Format'. A compressed image file format best suited to diagrams and images with a restricted colour range. In GIF compression, the colour range of the image is first reduced to 256 colours or less, then the remaining information is further reduced in size by a scheme known as LZW (Lempel-Ziv-Welch) compression. LZW compression is lossless, which means that you can edit and re-save the image without degrading it, but the initial colour-space compression makes the format unsuitable for high-quality colour photographs. Note however, that the 256 levels in a black and white image can be indexed as colours, which means that the GIF format is suitable for high quality continuous-tone monochrome images. The LZW algorithm is patented by Unisys, and in countries where the patent still applies (including UK) it is illegal to generate GIF images using software which has not been licensed. The Unisys patent expired in the US in 2003, but in other affected countries, open-source software users should consider the PNG format as an alternative.

Gland: In underwater engineering parlance: an joint or interface which prevents water ingress; e.g., control gland, cable gland, etc. A sealing structure which permits passage through a bulkhead (vessel wall), usually a combination of machined metal or plastic parts and O-rings.

GTO: Gate Turn-Off Thyristor. A power switching device used to switch-off (quench) the current in a flash tube in order to achieve automatic or TTL flash exposure control. Now largely replaced by the IGBT.

Guide Number: A number used to represent the illuminating power of a photographic flash. When electronic flash is used, the camera shutter opens (i.e., the exposure begins), the flash fires, and the shutter closes; but because the flash duration is very short (and assuming relatively little ambient light), the 'shutter' does not control the amount of light falling on the sensor or film. Consequently, the level of exposure due to the flash can only be controlled by the lens aperture (f-stop) setting. In metric countries, the flash guide number is the aperture setting required for ISO 100 film when the flash to subject distance is 1m (in North America, the guide number is the 100ASA aperture setting for a distance of 1 foot. Divide the American guide number by 3.28 to get it in metres). Using the Guide number G, the required aperture setting for any ISO speed and distance (in air) is obtained thus:
This equation is used to produce the guide table which is sometimes attached to the flash.

HAD: Hole Accumulation Diode. Diode structure used in some types of CCD image sensor.

Hand-wheel (nut female, bolt male ): A threaded nut or bolt which can be tightened by hand, usually in the form of a knob with a gripable (knurled) surface, or with protrusions (wings). hand_nuts

Handy: Quasi-English term (malapropism) used in Germany to mean 'Mobile Telephone', and used in other countries to mean 'hand-held'. In native English idiom, handy simply means 'useful'.

Helical Scanning: The problem in recording video signals onto magnetic tape is that a video signal has enormous bandwidth in comparison to an audio signal. This means that the tape must move at a very high speed relative to the recording head. Tape in an audio cassette moves at 47.625mm/sec (1+7/8 ips). To record analog video, the tape would need to move at about 5 - 10m/sec (11 - 22mph) depending on the required picture quality, which is impractical.. The solution is to mount recording heads on a drum, which is set at a slight angle to the path of the tape and spins rapidly while the tape moves by slowly.. The result is that the recording heads execute a helical path relative to the movement of the tape, and thereby write the video information at high speed as a series of diagonal stripes. All modern video-tape recorders use helical scanning, and the technique is also used to write CD-quality audio on to tape in DAT (digital audio tape) machines.

Hi-8: High-band Video 8.

HID (Lamp): High Intensity Discharge Lamp.

High-band video recording: Analog video signals are written onto tape as a frequency-amplitude modulated (FAM) radio-frequency signal. The frequency modulated (FM) part represents the luminance (brightness) information, and the amplitude modulated (AM) part represents the chrominance (colour) information. Early systems used a relatively low-frequency range for the FM, but as the sizes of the magnetic particles in the tape became smaller with improvements in the technology, higher frequency recordings became possible, with consequent improvements in the available video bandwidth. This led VTR manufacturers to release high-band versions of their recorders (e.g., S-VHS and Hi-8). The high-band machines can record and play the earlier tapes in the low-band format, but the old low-band machines cannot play the high-band tapes, and cannot record or erase them properly because higher magnetic field-strengths are required. A high-band machine can however, make a low-band recording on a high-band tape for playback in a low-band machine.

histo Histogram: In digital photography; a graph of the number of pixels occupying each possible brightness level. Experienced picture editors learn to look at the image histogram and assess the 'health' of the image in terms of what it will look like when printed. It there are serious gaps in the histogram (brightness levels not used or 'not populated') the picture will not be able to give the impression of continuous tonal gradation, and the print will appear 'posterised', ie., made out of discrete blobs of colour with obvious boundaries between them.

Hot Shoe: A camera accessory shoe with electrical connections for a flash unit. Different manufacturers use different connector pad layouts and electrical signals. A standard accessory shoe without any electrical connections is sometimes also erroneously called a hot-shoe, so that, for example, Sea & Sea makes a flash arm to fit on the accessory shoe and calls it a "hot-shoe mini-arm". hotshoe_nikon
hot shoe
Olympus hot shoe
hot shoe

Housing (underwater): That part of an underwater camera or other equipment in which the waterproof seals are located. A casing or box with waterproof seals, designed to contain a camera or other equipment in such a way that it can be used underwater.

Hydrophone: Underwater microphone. Because air is an easily compressible medium, but water is not; sound waves (pressure waves) travelling in water involve small displacements of the water molecules, whereas sound waves travelling in air involve relatively large displacements of the air molecules. Consequently, if you put an ordinary microphone in a box, the sound waves in the water won't move the walls of the box very much, won't compress the air in the box very much, and so won't move the diaphragm of the microphone very much. The solution is to have a specially designed microphone in which the diaphragm is actually in contact with the water and is sensitive to very small displacements. hydrophone
Bulkhead mounting hydrophone.

Hyperfocal distance (of a lens): The distance setting which gives the greatest possible range of distances at which objects appear to be in focus. Good lenses have means of indicating the depth of field for a given aperture setting. The hyperfocal setting for a given aperture is obtained by placing the most distant limit of the depth of field range on ∞. The corresponding 'hyperfocal distance' is the actual distance setting when the hyperfocal criterion has been met. Note that wide-angle lenses have an extremely large depth of field, and therefore, unless being used for extreme close-up work, are best set to the hyperfocal distance and left alone (refocusing is only necessary if the aperture is changed).

Hz, Hertz, c/s: Hertz is the unit of frequency, and is synonymous with 'cycles per second' (c/s). The unit was established in commemoration of the work of Heinrich Hertz, whose scientific experiments in the late 19th century were the basis of radio telecommunications).

ICS: Ikelite connector system.
ICS2: A 2-pin wet connector used with early Ikelite manual flash units.
ICS4: A 4-pin O-ring sealed connector used with early Ikelite automatic flash units.
ICS5: A 5-pin O-ring sealed connector used with current Ikelite TTL flash units and video lights.

ics2_pl ics2_sk ICS2 Connector
(3rd pin is for reversal prevention only)
ics5_pl ics5_sk ICS5 connector

IDC: Insulation Displacement Connector.

IGBT: Insulated Gate Bipolar Transistor. A general purpose power switching device, which finds application in photography as the series control (tube switch-off) element in automatic and TTL flash units. The IGBT control circuit is nowadays preferred over the old Vivitar GTO (Gate Turn Off) Thyristor circuit, because it can reset ready to fire in about 30ms (as opposed to 100ms for the GTO), and so gives better performance in systems which use pre-flashes for exposure evaluation (i.e., with digital cameras).

ikelite: The founder of the company was Ike Brigham, and the first product was an underwater light, hence "ikelite".

i.Link: Digital video interface (connection between camera and editing equipment, monitor, computer, etc.) specified in document IEEE 1394. Also known as 'FireWire'. Data transfer speed up to 400Mbit/s.

Inch: The Metric and American measurement systems were finally made compatible in the 1960s by a small adjustment to make 1" = 25.4mm exactly. This made it possible for any machine-tool to manufacture exactly according to either system by the inclusion of a 127-tooth gear in the screw-cutting gearbox.

Interpolation: Changing the number of samples in a data set (e.g., pixels in an image) by estimating the values for intermediate points for which no data exists.

Inverter: A device which produces a high voltage output from a low voltage source (sometimes also called a DC to DC converter). In portable photographic flash units, an inverter is used to charge a high-voltage capacitor from a battery, which is why the internal circuitry of a flash gun is dangerous even though the batteries can be handled with impunity. The inverter in a flash unit is an oscillator, which usually runs at a frequency in the audible range and so makes a whining noise.

IP rating:Ingress protection rating to BS EN 60529: 1992.

IR, Infrared: Beneath red. That part of the electromagnetic spectrum (light) which lies just below red in frequency (ie., of longer wavelength than red).

Iris: Synonymous with 'aperture', but now somewhat old-fashioned; the variable diameter hole used to control the amount of light passing through a lens. See f-stop.

IS: Image Stabilisation.

ISM: Industrial, Scientific, and Medical.

ISO: International Standards Organisation.

ISO-9660: The basic standard for recording computer data files onto Compact Disc. ISO 9660 allows only for upper case 8.3 type filenames, using the characters A to Z, _ (underscore) and - (minus or hyphen). Because of the limitations of ISO 9660, various extensions to the standard exist to permit the use of long filenames and more characters, but these extensions are not compatible across the various computer platforms (Microsoft, GNU/Linux, UNIX, Macintosh). Consequently, if you want to create a disc which can be read by all current computer systems, you must stick to plain old ISO 9660.

ISO film speed: The method of film speed (sensitivity) classification developed for the American Standards Association (ASA) and finally adopted by the International Standards Organisation (ISO). The scale is such, that if the ISO speed number of the film is doubled, the sensitivity of the film is doubled, eg., if for a given lighting situation, you change from ISO 100 film to ISO 200 film, you must either close down the lens aperture by one stop, or double the shutter speed.

Joule, J: The unit of energy. 1 Joule = 1 watt second.

JIS: Japanese Industry Standard.

JPEG: Joint Picture-Expert's Group. A full-colour image file format with variable compression. JPEG compression is always lossy (there is no such thing as a lossless JPEG), and so an image should only be converted to this format when all of the editing has been finished. Lightly compressed JPEG images are only distinguishable from the uncompressed original by careful comparison under high magnification. Heavy compression (all too prevalent on the web) produces very small files, but the quality is awful. It is better to keep picture sizes small than to display large images with obvious compression artifacts. See also EXIF.

K, Kelvin: The unit of absolute temperature, ie., degrees above absolute zero. Named after Lord Kelvin, who was the first to show that there is a degree of coldness below which it is not possible to go. 0K = -273.16°C. When a temperature is expressed in Kelvin, the ° symbol is not used, ie., eg., 0°C = 273.16K. See also Colour Temperature.

K: Potassium (kalium).

Lanyard: A short piece of rope or line used as a handle or tied-on to some other object. A wrist-lanyard is a loop of thin rope with a sliding toggle, which may be tightened around the wrist to secure the object (usually a torch or a camera) attached to it. When SCUBA diving, it is better to secure a camera by means of a wrist-lanyard than by using a neck-strap, since the mouthpiece must be removed in order to take a neck-strap on and off.

LCD: Liquid Crystal Display.

LED: Light Emitting Diode.

Lexan: An extremely tough polycarbonate plastic, usually transparent, used to make underwater housings. 'Lexan' is a trademark of the General Electric corp. See polycarbonate.

LZW: Lempel, Ziv, Welch. A fast lossless data compression algorithm patented by Unisys (see also GIF and TIFF).
Ref: "A Technique for High Performance Data Compression", Terry A. Welch, IEEE Computer, Vol 17, No 6, 1984, p8-19.

M: Prefix used to indicate a screw-thread conforming to the ISO (International Standards Organisation) Metric system (60° pitch angle). A metric thread specification includes a nominal diameter in mm and a pitch distance (i.e., distance in mm between adjacent turns of the thread), e.g., M6×1 = 6mm diameter, 1mm pitch; M8×1.25 = 8mm diameter, 1.25mm pitch. The pitch distance is often included because each diameter is available with a range of pitches from coarse to fine; but if the pitch distance is omitted, it is usually assumed that the coarsest thread is implied: e.g., the thread of a Nikonos connector is M14×1, specifying it as M14 will not do, because the coarsest 14mm thread is M14×2. Camera filter threads are an exception to the 'always state the pitch unless it's the coarsest one' rule however because, apart from the deliberate awkwardness of some manufacturers, the filter thread pitch is 0.75mm. To find the tapping hole size for a metric thread, simply subtract the pitch from the nominal diameter; e.g., for an M6×1 socket, the tapping drill is 5.0mm. Note when measuring threads, that a screw is always slightly smaller than the nominal diameter, and a socket (threaded hole) is slightly larger. This difference (usually a few 10ths of a mm) is necessary to ensure that the parts will fit together and turn smoothly.

Macro: Larger than life. Strictly, a macro photograph is one where the image recorded on the film is larger than the subject. The term is used loosely however, to describe an image which can easily be printed or projected `larger than life'. Cameras are usually adapted for macro photography by adding a lens extension tube or a supplementary lens, or in the case of an SLR, by fitting a lens which can focus at very short distances.

Macro lens: An extreme close-up lens. This may be a complete camera lens, or a lens attachment which screws into the filter ring or bayonet mount of the main camera lens. A macro lens is usually associated with a ratio such as 1:1 or 1:2 etc. The first number is the relative size of the image on the film, the second number is the relative size of the subject, so read a:b as "a on the film, b in real life". SLR macro lenses are usually focusable from infinity down to 1:1 (life-size on the film), or 1:2 (half life-size on the film). Nikon refers to its SLR macro lenses as `micro' lenses, e.g., `60mm AF Micro-Nikkor'. A removable lens can be converted into a macro lens by the addition of an extension tube.

Macro port: A flat port, ie., a plane glass or plastic optical window. The term arises because a flat port is most useful with macro lenses.

MEK: Methyl-ethyl-ketone. CH3-CO-C2H5. A volatile, flammable, chemical solvent. Used in the printing industries.

Memory Effect (of Ni-Cd cells): Early Nickel-Cadmium cells had a reputation for losing capacity if they were not discharged fully during each cycle of use. This was dubbed 'the memory effect', i.e., the cell remembered that you didn't use it to the full and changed accordingly.  Improved design eliminated the problem, but the myth persisted.  NiCd batteries are now banned from sale to ordinary consumers in Europe under the Restriction of Hazardous Substances (RoHS) legislation (they are still permitted in emergency lights for their tolerance of continuous trickle charging).  The modern replacement (NiMH) has little or no memory effect, but cannot stand being overcharged and so requires a complicated charging system.

Meniscus Lens: A lens which is concave on one side and convex on the other. If the curvature of the convex part is greater than that of the concave part, the lens is a magnifying glass. If the concave part has greater curvature than the convex part, the lens is a de-magnifying glass. Inexpensive screw-in close-up lenses are of the meniscus type, this being a good compromise between optical quality (freedom from aberrations) and cost. Close-up lenses constructed as achromatic doublets are considerably superior, but much more expensive.

MM2: Abbreviation for Motormarine Mk II (A Sea & Sea camera).
MM2 Bayonet: The Motormarine II bayonet mounting system for supplementary lenses.

Molykote: Dow-Corning trade name for O-ring lubricants. Molykote 111 is the industry standard silicone grease, a highly water-repellent non-toxic lubricant, often re-packaged into small tubs by underwater housing manufacturers. Molykote FS1292 is fluorosilicone grease, a special formulation designed to reduce the problem of swelling due to grease absorption which occurs when silicone grease is used on silicone rubber O-rings. Despite the name, Molykote lubricants do not contain molybdenum disulphide.

MTF: Modulation transfer function. A factor between 0 and 1 indicating the extent to which contrast is reduced when an optical system is projecting a pattern at a particular resolution. Also known as Spatial Frequency Response (SFR)

Na: Sodium (natrium).

NBR: Nitrile Butadiene Rubber. A rubber with excellent resilience and chemical resistance. Preferred material for the manufacture of camera O-rings, usually black. NBR has only limited UV resistance, and so should be stored in the dark. The optimal lubricant for NBR is silicone grease.

NEX: New E-mount eXperience.  Sony interchangeable lens compact camera using the E-mount lens bayonet specification.

NiCd, "Nicad": Nickel-Cadmium rechargeable battery. NiCd batteries are no longer marketed for consumer applications in Europe due to RoHS regulations (Cadmium is no longer permitted in disposable items). See also memory effect.

Nikonos: Underwater film stills camera system, manufactured by Nikon; developed from the 'Calypsophot' camera designed by the Belgian engineer Jean DeWouters for the Calypso expeditions of Jacques Cousteau.

Nikonos grease: Petroleum gel (also marketed as 'Vaseline'). Suitable lubricant for NBR (black nitrile) O-rings, but somewhat inferior to silicone grease.

Nikonos connector: There are three generations of Nikonos connector: I (2 wires, used on Calypsophot, Nikonos I and Nikonos II), III (3, wires, used on Nikonos III and IV), and V (five wires, used on Nikonos V). Nowadays, only the 5-wire (TTL flash) connector is used.
nikv_pl Nikonos V (or 5) Connector nikv_sk

Note that the socket has three fixed pins and two spring-loaded retractable pins (this is for compatibility with the Nikonos III plug). Some manufacturers produce a variant with fixed pins in place of the retractable ones. This is not a true Nikonos connector, and a standard Nikonos V plug will be damaged if it is screwed into such a socket.

NiMH: Nickel metal-hydride (cell or battery). High-capacity rechargeable battery, free from toxic cadmium.

Nitrile rubber: see NBR.

NOS: New Old Stock

NTSC: 'National Television Systems Committee' - The colour television system used in the USA, Canada, and Japan.

Ohm, Ω: The unit of electrical resistance. A resistance of 1 Ohm will pass a current of 1 Amp when a voltage of 1 Volt is applied across it.

OEM: Own Equipment Manufacturer. Generally a supplier who builds finished products from sub-assemblies supplied by other manufacturers.

OK: Affirmative (Scottish: "Och aye!"): A light on a flash unit which illuminates briefly when a TTL-stop (quench) signal has been received. See TTL flash.

OLED: organic light-emitting diode.

OR: Abbreviation for 'O-ring'.

O-ring: The un-prepossessing rubber ring which keeps the water out of underwater housings is actually part of an extremely clever sealing system which becomes more efficient as the pressure difference between inside and outside increases. The cleverness lies not so much in the ring itself, but in the shape of the groove in which it sits, which is designed so that the pressure forces the rubber into the gap through which the water is trying to pass. Consequently, and perhaps surprisingly, an underwater housing is more likely to leak at shallow depths than at great depths. One thing which will disrupt the operation of an O-ring seal however, is dirt, which creates gaps and channels through which the water may creep. Good husbandry in underwater photography therefore, lies in the matter of cleaning the O-ring and the O-ring groove meticulously before closing the housing.

Oscillator: A device which undergoes a cyclical behaviour when supplied with energy. A pendulum is an oscillator. A weight on a spring is an oscillator. The term however is normally used to describe electronic circuits which produce an alternating current or voltage when connected to a battery or a DC power source. Oscillators are used in radios, TVs, watches, cameras, electronic flash units, computers, etc., etc., etc.

PAL: 'Phase Alternating Line' - The colour television system used in Europe, Australia, and New Zealand.

Parallax: The apparent difference in position of an object when seen from two different viewpoints. The problem with cameras having a viewfinder separate from the main lens (as opposed to reflex cameras) is that the main lens and the viewfinder see the subject from slightly different positions. This causes framing errors. A partial solution is to tilt the axis of the viewfinder, but a different amount of tilt is required depending on the distance to the subject. External viewfinders sometimes have a parallax compensation knob, which tilts the viewfinder according to a distance scale. Built-in viewfinders are usually set for correct framing at some specified distance, but may have parallax compensation marks in the framing window for use at short lens to subject distances. Single-Lens reflex cameras do not suffer from viewfinder parallax.

Pb: Lead (plumbum).

PC: PalmCorder, i.e., a Camcorder which can sit on the palm of the hand.

PC: 'Photo Co-ax.' Small circular connector used in conjunction with manual flash equipment.


PCB: Printed circuit board.

PCM: Pulse-code modulation. Digital audio recording mode used by tape recorders.

Pel: 'Picture element'. Now obsolete. The preferred term is 'Pixel'.

Phonetic Alphabet: A set of words which represent letters, used to facilitate the sending of messages via restricted-bandwidth communication systems (i.e., radio). The International Phonetic Alphabet is the approved set of such words, being chosen so that no two words sound similar: Alpha, Bravo, Charlie, Delta, Echo, Foxtrot, Golf, Hotel, Italy, Juliet, Kilo, Lima, Mike, November, Oscar, Papa, Quebec, Romeo, Sierra, Tango, Uniform, Victor, Whiskey, X-ray, Yankee, Zulu.

Photography Copyright:  Under law, it is the Photogropher who will own copyright on any photos he/she has taken, with the following exeptions
1. If the photographer is an employee of the company the photos are taken for, or is an employee of a company instructed to take the photos, the photographer will be acting on behalf of his/her employer, and the company the photographer works for will own the copyright.
2. If there is an agreement that assigns copyright to another party.
for more info look at the Photography Copyright fact sheet from UK Copyright Service.

PIC: Programmable Interrupt Controller.  A microcontroller (single-chip computer) used for controlling electronic equipment.  An 'interrupt' is a request for action, such as might be generated by a button-press or some other input.

Pixel: 'Picture element'.
of a colour display: a dot or a small area of a picture which carries the three attributes: hue, colour saturation, and brightness. Any picture can be made up of an array of 'dots' carrying these attributes and viewed from such a distance that the dots are no longer visible. In practice, pixels are usually composite entities; the pixels of a TV screen, for example, being made up of areas of red, green, and blue.
of an RGB image file (after decompression): a set of three binary numbers representing brightness values for red green and blue at a particular point in the image.
of a CMYK image file (after decompression): a set of four binary numbers representing amounts of cyan, magenta, yellow, and black ink to be applied at a particular point in a printed image.
of a camera: an individual light-sensing element. The number of pixels attributed to a digital camera is usually the total number of light sensing elements (red + green + blue). The camera pixel count is usually taken to be a figure of merit for image resolution (detail recording capability), but this figure is not directly related to the displayable pixels recorded in the output files.
For more information, see the article Pixels and resolution.

PMR: Professional Mobile Radio - e.g., the hand-held VHF/UHF transceivers used for emergency and ship-to-shore communications.

PNG: Portable Network Graphics. File format designed to get around the software patent on the LZW algorithm used in GIF compression (Unisys patent on LZW expired in the US in 2003, but may still apply in some European countries, including UK).

POA, £POA: Price on application; i.e., when someone asks for one we'll find out and let them know.

Polycarbonate: Tough plastic which can be used to make high-quality injection-moulded engineering components. Trade names: Lexan, Merlon, and Tuffak. Preferred material for sport-diving-depth underwater housings; also famous as the material used to make bullet-proof shields. Polycarbonate is attacked by chlorinated solvents, but can be cleaned using mild detergent and water or alcohol. Polycarbonate is liable to crack if drilled or machined incorrectly (specially ground drills are required). See also Lexan.

POM: Polyoxymethylene.

Port: Optical quality window in an underwater housing (by analogy with a ship's 'port hole').

Portrait Lens: Have you ever wondered why people look extremely ugly in passport photos (particularly the ones taken in photo booths)? The reason is that the picture has been taken from short-range using a wide-angle lens, causing the subject's ears to disappear behind the cheek-bones. To photograph a face in proper proportion, the camera must be a reasonable distance away from the subject (not possible in a photo-booth) and a lens of somewhat longer focal length than a normal (standard) view lens is required in order for the head and shoulders to fill the frame. For 35mm cameras, the best focal length for portraits is in the range 80 - 135mm, and lenses with focal lengths in this region are known as 'portrait lenses'.

Pre-flash: A short burst of light emitted from a flash unit just prior to the main flash. A pre-flash can be used for exposure evaluation: i.e., it can be used to make a trial exposure from which the correct exposure for the photograph is calculated. One or more pre-flashes can also be used to cause a portrait subject's pupils to contract, this being a somewhat dubious attempt to reduce the red-eye effect which plagues compact cameras (and which results from placing the light source too close to the camera lens). Some cameras, particularly when set to control proprietary slaves, may issue multiple pre-flashes during the course of an exposure evaluation.

PSD: Photoshop document. Adobe proprietary format for storing multi-layered images (ie., photo-montage in progress). PSD is the preferred image file format for Photoshop users; and given the widespread use of Adobe software in the photographic and publishing industries, it is also a perfectly good file exchange format. Note that the PSD specification changes with every whole number version release of Photoshop, but is backwards compatible; so you must have the latest version to be sure of being able to open any PSD file.

Pupil:Exit pupil: The point from which all rays of light appear to emerge from a lens. Entrance pupil: The point from which rays of light appear to emerge from a lens when it is reversed (turned around). For compound (multi-element) lenses, the precise location of the pupil is best found by looking at the manufacturers detailed specification (if available), but for practical purposes, it is where the iris appears to be when you look into the lens. Note that for an asymmetrical lens, the pupil appears to be in a different place depending on which side of the lens you examine, for which reason we talk of `entrance' and `exit' pupils. The entrance pupil is the one you can see from outside the camera.

PVC: 'Poly Vinyl-chloride'. A normally hard plastic, with good resistance to weathering and UV light, which can be made flexible by the addition of plasticisers. The un-plasticised version is called "UPVC". PVC is attacked by many industrial cleaning agents, especially chlorinated solvents, ketones (e.g., MEK), and furans (e.g., THF). Cleaning with agents other than mild detergent and water is not recommended.

Quench: Extinguish, put-out. The business of turning off the current in a flash-tube for the purpose of automatic exposure control, it being no easy matter to put the damned thing out once the discharge has begun. In TTL flash photography, the camera must send a 'quench' signal to the strobe (flash gun).

Quench tube: There are two ways of quenching a photographic flash. The modern method is to switch off the current in the tube using a series control element such as a GTO (gate turn-off) thyristor (often the ubiquitous Mitsubishi CR3JM), or better still an Insulated Gate Bipolar Transistor (IGBT). The old fashioned method is to dump the residual charge in the capacitor by shorting it with a special trigger tube (thyratron) with a very low on-resistance, called a "quench tube". The quench tube is a small Xenon tube with a large electrode area and a small inter-electrode distance. Quench tubes waste the residual energy and require the system to be recharged from scratch after a controlled flash. Switching off the current with a GTO or IGBT conserves the residual energy and so shortens the recycle time after a controlled flash. Consequently, quench tube systems are obsolete (and very annoying to use).

Raster: The rectangular area of illumination formed on a TV screen by the scanning process. An array of lines or dots of varying brightness which, when viewed from a suitable distance, produces the illusion of an image. Originally a German word for a printer's half-tone screen. Probably first used in connection with 'electrical photography' by Arthur Korn in his 1907 book "Elektrische Fernphotographie (electrical distance photography)" (source: Richard F Lyon, Foveon Inc. private communication 2004).

Rasterisation: The conversion of a vector-based graphical representation into an array of pixels.

Ready: A flash unit is ready to fire once the main storage capacitor has been charged to a voltage at which the tube can be expected to trigger reliably. This situation is usually communicated to the user by lighting a neon lamp or an LED, and to the camera by sending a signal along the sync. cable which causes the camera to adopt a shutter speed at which flash (X) synchronisation is possible. Thus a flash unit has a 'ready light', and a flash cable or connector (with certain exceptions) carries a 'ready signal'. Note that the ready light or signal is asserted when the flash unit is triggerable, but it does not imply that the flash is ready to give full output. If a flash unit has voltage regulation, you will hear interruptions in the whining noise from the inverter when the capacitor is fully charged. If the flash unit has no voltage regulation, the inverter will run continuously, and in this case, as a rough rule of thumb, you should wait about twice the time it takes for the ready light to come on after a full light burst to ensure that the capacitor is fully charged.

Rebikoff port, Rebikoff corrector: A lens which corrects for refraction at the air-water boundary, characterised by a flat surface in contact with the water and a concave surface in contact with the air. The Rebikoff corrector is afocal, and produces less chromatic aberration than a dome port, but is more expensive to make.

Reciprocity, Principle of: Photographic exposure is defined as: light intensity multiplied by exposure time, ie., I × t. It follows that a given amount of exposure can be achieved by using a high intensity and a short time, or a low intensity and a long time, or somewhere in between, provided that the required product I × t is obtained. This is known as the (photographic) principle of reciprocity (increase one, reduce the other). The principle of reciprocity only holds true however if the recording medium has a linear (proportional) response to light intensity, and this is not always the case. Film is reasonably linear, but needs additional exposure time at very low light levels. CCD sensors become non-linear at very high brightness levels. When the exposure setting (as determined using a light meter, for example) has to be adjusted to compensate for very low or very high brightness levels, the recording medium is said to suffer from reciprocity failure.

Rectilinear Lens: A lens which reproduces right-angles and straight lines correctly.

Red-eye: If a flash light source is placed very close to a camera lens, a direct reflection from the blood vessels in the back of the eye will cause a portrait subject to appear to have illuminated red pupils. The proper solution to this problem is to move the light source away from the lens (i.e., to use a flash unit separate from the camera). The compact-camera solution however is either to shine a light into the subject's eyes, or to fire one or more pre-flashes; the object of the exercise being to cause the subject's pupils to contract, thereby 'reducing' (but not eliminating) the red-eye effect. All too often, a pre-flash will cause the subject to blink; and we may then presumably console ourselves with the observation that there is definitely be no red-eye problem when photographing people who have their eyes closed.

Refraction: The bending of rays of light as they pass through the boundary between two optical media, e.g., from air to water, or from air to glass. Only rays which are not perpendicular to the surface are bent.

Refractive Index: A measure of refracting power, defined as the velocity of light in vacuum divided by the apparent velocity of light in the medium. Water has a refractive index of about 1.33, which means that light travelling through water has an apparent velocity (phase velocity) of about 0.75 × the true speed of light.

Resistance, Resistor: All electrical conductors (except superconductors) have some resistance, which means that they convert electrical energy into heat. A resistor is simply a device for which the property of resistance is accurately characterised. Resistors are useful because they obey 'Ohms law', ie., the current which flows in a resistor is exactly proportional to the voltage applied across it.

Resolution (resolving power): The detail recording capability of an image acquisition system (camera, lens, etc.). Resolution is usually expressed in lines per unit length, e.g., colour film can resolve about 110 lines/mm, the UW-Nikkor 15mm f/2.8 lens can resolve 73 lines/mm at the centre of the image at f/8. The number of pixels in an image is not a measure of resolution, but it is related of the maximum amount of detail which can be represented.

Reversal Film: Positive (i.e., slide) film. Film in which the original negative image is reversed during the development process to produce a positive transparent image.

RF: Radio Frequency

RGB: Red, Green, Blue. The 'additive' primary colours. The colour-space representation used by TV and Video monitors.

RMS: The square root of the mean of the squares. Of an alternating voltage or current: the equivalent steady (direct) voltage or current which will have the same heating effect.

RoHS: Restriction on the use of Hazardous Substances. European legislation.

RX: Receiver, Receive.

SCART: "Societé de Constructeurs d' Appareils Radio recepteurs et Téléviseurs". 21 pin interface connector for direct audio + video connection between TV and VCR, DVD, etc. Also known as a 'Péritel' or 'Euroconnector'.

SCUBA: Self-contained Underwater Breathing Apparatus (as opposed to diving equipment supplied with air from the surface). Term normally applied to diving equipment which supplies air from a tank by means of a demand (suction operated) valve. Invented in practical form in the 1940s by Jacques Cousteau and Emile Gagan. Cousteau originally dubbed the device an "Aqualung"; but the British Navy decided that this sounded like an unpleasant disease, and insisted on renaming it "SCUBA".

Sealock connector: Also known as a Nelson connector. A 2-pin wet-connector, ie., a connector which can be pulled-out and plugged-in underwater. Used for flash synchronisation on the Sea & Sea Motormarine Mk1. Also used as a telephone connector on full-face diving masks.
sealockSealock connector

Sea & Sea connector: A 4-pin waterproof connector used with the Sea & Sea TTL flash system.

SECAM: 'Sequential Couleur avec Memoire' - The colour television system used in France, French overseas territories (not Canada), and Russia.

Shore hardness, Sh: A hardness scale from 0 - 100 used to express the deformability of elastic materials (rubbers). Commonly available O-rings have a hardness of around 70sh. The O-rings used in Nikonos cameras are 60sh. Using too soft an O-ring may result in failure of the seal by extrusion (pushing-out). Using too hard an O-ring will result in extreme stiffness, and potential failure due to excessive wear.

Shutter lag: The time interval between pressing the shutter-release button and the opening of the camera shutter (often several tenths of a second). Some delay is inevitable in SLR cameras (which must flip a mirror out of the way); and in cameras which use the pre-flash method of flash exposure evaluation (and must therefore allow time for the flash unit to recover from the pre-flash). Auto-focus systems exacerbate the problem enormously, expecially when working in low-light conditions, and there is considerable advantage in having the option to lock the focus, or to use manual focus.

Slave flash, slave sensor: A slave flash is a flash unit which is triggered to fire by the light output from another flash unit (the master flash). The light from the master flash is detected by a sensor, usually a phototransistor, to generate an electrical signal in the event of a sudden increase in light level. The slave sensor can either be built-in to a flash unit with a 'slave mode', or it can be a separate unit which triggers a flash via its sync. cable.

Slot-stay: Camera tray with captive tripod screw in a slot, arranged so that the camera or housing can be pulled back against an anti-rotation ridge.

SLR: Single-Lens Reflex Camera. A camera which uses a mirror to redirect the image which will be projected onto the film into the viewfinder. The mirror flips out of the way when the button is pressed to take the picture.

SLT: Single-lens semi-transparent-mirror reflex camera (SLR in which the mirror stays in place during shooting).

Sn: Tin (stannum).

Snell's Law, Snell's Window:

Sound: Energy transmitted by longitudinal pressure waves in a medium such as air or water. The human hearing range is generally considered to run from about 15Hz (cycles per second) to 20KHz, although this view is somewhat misleading. Humans generally feel rather than hear low frequencies, and the auditory perception of sounds below about 50Hz is mainly due to the production of distortion products (harmonics) by the ear itself. Only young children can hear up to 20KHz, and by middle age, 12 to 15KHz is more realistic (8KHz if you were in the habit of listening to loud music as a teenager). The intelligence in human speech is contained in a band from about 300Hz to 4KHz. Radio communication systems (PMR, Ship to Shore) often limit the bandwidth to 300Hz to 3KHz, but this makes it necessary to spell out some words using the Phonetic Alphabet, there being, for example, no way to distinguish between the sounds for 'f' and 's'.

Sound Velocity: The velocity of sound in dry air at 1Bar is given approximately by the formula:
v = 331.45 + 0.6T metres / second   (where T is the temperature in centigrade).
A figure to remember for general reckoning is: "about 340m/s".
For estimating the distance ot thunderstorms: "about 3 seconds per kilometre".
If you need an accurate value see:
"The variation of the specific heat ratio and the speed of sound in air with temperature, pressure, humidity, and CO2 concentration" Owen Cramer, Journal of the Acoustical Society of America, May 1993, Volume 93, Issue 5, pp. 2510-2516.
The velocity of sound in sea water is approximately 1500m/s.
The velocity of sound in fresh water is approximately 1435m/s.

S/PDIF: Sony / Philips Digital Interface Format. Digital audio interface standard, for transferring audio signals between CD players and DAT machines, etc., without passing into the analog domain.

Specular Reflection: Direct reflection, i.e., the glinting reflections from white and shiny objects which cause bleaching of the highlights in a photograph. The problem of specular highlights can be reduced by using a diffuse light source, i.e., a light source of large area, rather than a point source.

Speedlight: As opposed to bulb flash, a slightly old-fashioned name for electronic flash. Term still used by Nikon and Canon, and a pleasant alternative to the American term "strobe" which avoids using the words "gun" or "blitz".

SS, Stainless Steel: An alloy of Iron, Chromium, Manganese, Silicon and Carbon (May also contain Nickel and Molybdenum). The alloying elements, particularly Chromium, react with oxygen and water to form a thin film of oxides and hydroxides, which prevents further chemical attack. All stainless steels contain at least 10% Chromium. The most commonly available grades of stainless steel are denoted 'A2' and 'A4'. A2 stainless (also known as type 304) shows superficial rusting in the presence of sea water. A4 stainless (aka type 316) is completely resistant to sea water corrosion and is the preferred grade for marine applications.

Stop: Photographers talk loosely of relative illumination level in 'stops'. A change of exposure level of one stop is a doubling or halving of the shutter speed, a doubling or halving of the ISO/ASA film speed, or a doubling or halving of the area of the lens aperture, i.e., in the latter case, a change of one f-stop, from which the term is derived. A change in illumination of 1 stop is the same as a change in illumination of 1 'Exposure Value (EV)', this being the correct term.

Strobe: The terms `Strobe Light' and `Flash Gun' are interchangeable. The former usage is North American, the latter originates from the UK. It is best to avoid the common British term when travelling outside the UK since, if some official, who may not speak English very well, asks what you've got in your bag, you don't really want to use any phrase with the word 'gun' in it.  In British English however, 'strobe' carries the connotation of repeated flashing (it is a shortened form of 'stroboscope'), so use of the term can cause confusion. International English alternatives are 'photo flash', 'flash unit' and 'speedlight', but not 'Blitz' (lightning), which has the additional British English meaning 'wartime aerial bombardment'.

Substrobe (SS-): Ikelite's name for an underwater flash unit.

S-VHS: Super VHS, i.e., High-band VHS.

S-Video: 'Separated Video', ie., two signals in two cables, one being the video, blanking and synchronisation, the other being the chrominance (colour) signal.

Sync., synchronisation (flash): see X-sync. The cable between the camera and the flash unit is called a 'sync. cord'.

Sync., synch., synchronisation (television): A television picture is composed of lines, of variable brightness and colour. A television monitor writes horizontal lines on the screen, starting at the top left-hand corner and travelling downwards. Synchronisation signals are required to tell the monitor when to start a new line (horizontal sync.), and when to flip back to the top for the start of a new field (vertical sync.). The sync. signals are in the form of electrical pulses which are added to the video (picture) signal.

System compact camera: A camera which has interchangeable lenses but which is not an SLR.  Eg., Nikonos, Olympus PEN.  

Targeting Light: A light, usually built into a flash unit, used to give an accurate indication of the direction in which the flash unit is pointing. The targeting light should be designed to be weak in comparison to the intensity of the flash, so that its patch of illumination does not show up in the picture. When using a torch as an auxiliary targeting or focusing light, beware that some modern torches are far too bright for the job and may ruin your pictures.

TBA: Abbreviation: 'To be announced'.

TBC: Time-base corrector. Helical-scan analogue video recordings (Video8, Hi-8, VHS, etc.) are affected by minor speed fluctuations (wow and flutter) in the tape-transport mechanism. This causes timing jitter in the horizontal synchronising signals, and leads to wobbly verticals in the picture. A time-base corrector digitises the video signal and stores the lines temporarily so that they can be released at exactly timed intervals, causing a dramatic improvement in picture quality.

Telecentric lens: A problem with digital camera sensors is that they separate colours by placing an array of colour filters in front of an array of light sensitive cells. This means that a light ray striking the sensor at an oblique angle can leak across into an adjacent cell, causing a degradation of the colour analysis (i.e., 'colour crosstalk'). The problem is minimised by placing the exit pupil of the lens at infinity (in front of the lens), in which case the lens is said to be image-side telecentric. The result is that light rays emerge substantially parallel to the optical axis.
     The lack of telecentric design is one reason why old wide-angle lenses designed for film cameras tend to give poor results with digital cameras. Standard, portrait and telephoto lenses generally do not to send severely off-perpendicular rays to the sensor, and so old lenses of longer focal length are often still capable of good performance.

THF: Tetra-hydro-furan. A volatile, flammable, chemical solvent; used as the basis for PVC and ABS solvent-weld adhesives.

TIFF: Tagged Image File Format. A lossless full-colour image file format with optional LZW compression. TIFF files can be opened, edited and re-saved without degrading the image. The TIFF specification allows for up to 48 bits-per-pixel, CMYK or RGB colour, and covers just about every requirement for electronic representation of single-layer still images. TIFF is the format of choice for exchanging and archiving high-quality photographs.

TLC: Technical Lighting Control - Aquatica trade name for a range of lighting support brackets and adapters.

Thyristor: An electronic switching device which (among manifold other uses) is used to switch off the current in a flash tube for the purpose of TTL exposure control.

Transformer: An electrical device for changing from one AC voltage to another, usually consisting of two or more coils of wire wound on a common core of magnetic material. Used in electricity substations for transforming from the distribution-grid voltage to the local mains voltage. Used in power supplies and battery chargers for transforming from the mains voltage to the low voltages required by electronic equipment. Used in conjunction with an oscillator, for generating high voltages from low voltages inside TV receivers and flash units. A device which comes in all sizes and has a huge number of uses (the smallest transformers are about the size of a grain of rice, the largest are delivered on huge low-loaders with police escort).

Tray: The metal plate or bar which screws to the bottom of a camera or housing so that a supporting arm for a flash unit or lamp may be attached to it. Also, the metal or plastic plate on which a camera sits inside an underwater housing. Sometimes also called a 'saddle', a 'stay', a 'rail', or a 'bracket'.

TTL flash: 'Through The Lens' flash metering. Another term, which never caught on, is 'Off The Film' (OTF) flash metering. It is basically an improved form of automatic flash control which takes account of the film speed, lens aperture setting, and any filters which might have been fitted to the camera.
     Although the full burst of energy from a flash unit may seem instantaneous, it usually last for about 1.5 thousandths of a second (~1.5ms). The light output can therefore be controlled by reducing the time duration of the flash, ie., by switching off the current in the tube before the storage capacitor is fully discharged. A TTL flash unit is one which can accept a signal from the camera telling it to 'switch off now'. The important thing to understand about this system is that the flash unit plays no part in calculating the exposure; it simply accepts the instruction to switch off at a point determined by the electronics in the camera, and will usually issue some kind of error signal if no such instruction is received. In 'OTF' metering, the camera determines the exposure by measuring light reflected from the film and summing (integrating) it over time. It compares the amount of light received against a level determined by the ISO (ASA) setting, and issues a stop (quench) signal at the appropriate point. If you reduce the amount of light falling on the film by stopping the lens down or fitting a filter, the camera will take longer to issue the stop signal. The camera can therefore compensate for all relevant variables until a point is reached when there is insufficient light to complete the exposure; in which case no stop signal is sent, and the flash unit issues a warning (Sea & sea strobes have a green 'TTL OK' light which comes on if a quench signal is received. Nikon and Ikelite strobes flash the red 'ready' light if no quench signal is received.). It follows, that a TTL flash system requires a TTL capable flash unit and a TTL capable camera; and less obviously, that the two units must be compatible (ie, they must conform to the same electronic interface standard).
     With the advent of digital photography, the definition of 'TTL flash' has expanded to include the 'pre-flash' method of exposure evaluation. It is difficult to devise a good 'off the film' metering system when using a digital sensor, because the sensor has a shiny rather than a matte surface. The solution is to issue a pre-flash, determine the exposure level obtained by averaging information read from the CCD, and use this to calculate the required duration for the main flash. The same system of start and stop (trigger and quench) signals can be used to control the flash unit, provided that the flash unit can recover and be ready to fire again in the short (less than 100ms) interval between the pre-flash and the main flash. A disadvantage of the pre-flash system is that it increases shutter lag (i.e., the time between pressing the button and taking the photograph), and it may cause portrait subjects to blink. Improvements in flash technology (the use of an IGBT as the switch-off device), can reduce the pre-flash - main flash interval to about 30ms.

TTL metering: Light metering for the determination of photographic exposure can be carried out in two ways: either by measuring the ambient light; or by measuring the light reflected by the scene. The ambient light method is best for reproducing the overall ambience, whereas the reflected light method is best for fitting the range of brightnesses encountered to the contrast range of the recording system (film or sensor). Nowadays, the reflected light method is almost universal because it can be done 'through the lens' i.e, using a light sensor inside the camera, or using the image sensor itself. This has the advantage that the reading can be used to adjust the exposure settings automatically, and it automatically compensates for the light loss which occurs in lenses and filters. No exposure metering system is perfect however, especially if the scene has very high contrast; for which reason good cameras have exposure compensation controls and allow the photographer to examine the image histogram.

TX: Transmitter, Transmit.
TX/RX: Transmit-Receive.

ULCS: Ultralight Control Systems. Manufacturer of lighting brackets.

U-Matic: Sony analog video cassette recording system using helical scanning and 3/4" magnetic tape. First appeared mid 1970s. Once favoured by industrial training departments, drama schools, etc., but now obsolete.

UNC: Unified Coarse. An American system of screw-thread sizes based on inch dimensions but with the same 60° pitch angle as the European metric thread system. The most common UNC thread used in photography is 1/4UNC (the standard tripod socket thread) more completely specified as 1/4UNC20, i.e., a 1/4" (6.35mm) diameter rod threaded at 20 turns per inch (tpi).

UNEF: Unified Extra-Fine.

UNF: Unified Fine. American fine thread system with 60° pitch angle. Smaller UNF sizes are numbered, whereas larger sizes are known according to the nominal diameter. E.g., #10UNF32 = 0.19" diameter rod threaded at 32 turns per inch. 1/4UNF28 = 1/4" rod threaded at 28tpi.

UPVC: Un-plasticised poly-vinyl-chloride: a hard engineering plastic. See PVC.

UTC: Universal Time Co-ordinate, i.e., the astronomical time at zero longitude. Formerly known as GMT (Greenwich mean time).

UV, Ultraviolet: Beyond violet. That part of the electromagnetic spectrumwhich lies beyond the violet end of the visible spectrum, ie, of shorter wavelength than violet, and invisible to humans. The problem with UV is that it will pass through camera lenses, film and CCDs are sensitive to it, but lenses are not designed to focus it. The result is a haze or fuzziness in pictures taken outdoors. The solution is to fit a UV filter (sometimes called a UV-390) ie., a filter designed to cut off at a wavelength of 390nm (nanometres), the end of the visible spectrum. Some photographers fit a UV filter also to protect the main camera lens from scratches. A UV filter is not needed underwater.

UW: Underwater.

UWCC: Underwater Colour Correction (filter). An optical filter designed to introduce a colour cast having the complementary colour of the cast introduced by the water. The colour cast introduced by so-called 'blue' water is somewhere between blue and cyan, hence the complementary colour is somwhere between yellow and red, i.e., orange. The complementary colour for green water is magenta. urpro_disc
filter disks

VCR: Videocassette recorder. See also VTR.

VHS: Video Home System; i.e., a video recording format intended primarily for domestic use. Analog recording system using helical scanning and 1/2" magnetic tape.
VHS-C: VHS Compact. Small-size VHS cassette intended for camcorders. Can be played back in full-size VHS machines by means of an adapter.

Video: Latin: 'I look'. An electrical signal which represents a picture; as opposed to television: a modulated radio signal which represents a picture. A video standard describes the details of the basic electrical signal which passes along local cables. A television standard is an extended video standard containing details of a method by which video signals and accompanying sound can be transmitted by radio and other long-distance communication methods.

Video-8: Analog video recording system using helical scanning and 8mm magnetic tape.

Volt, Voltage, V: The unit of electrical pressure (strictly: pressure difference), named after Henry Volta who invented the 'Voltaic pile' (nowadays known as the battery). Since a voltage is a pressure difference, it must always be measured relative to something, and that something is usually a common connection called 'ground', 'earth', 'common', 'chassis', or 'mass'.

VTR: Videotape recorder. A videocassette recorder (VCR) can also be called a VTR, but a video recorder which uses open-reel tapes cannot be called a VCR.

W: Double (because it's a "double U"). Used as a prefix of suffix in Japanese product names, eg: "W-Stay" = dual flash lighting tray; "Lens-holder W" = dual lens holder.

W: Tungsten (wolfram)

Watt, W: The unit of power, ie., the amount of energy dissipated (used) or delivered in unit time. 1 Watt = 1 Joule per Second. Electrical power, in Watts, is also the product Current × Voltage (P = IV), eg., a 12V light-bulb which consumes 60W must draw a current of 5A.

Wavelength: see Electromagnetic radiation.

WEEE: Waste Electrical and Electronic Equipment directive. European legislation.

Wet Connector: A connector which can be un-plugged and re-connected underwater. Sealock (Nelson), E/O (Electro-Oceanic), and ICS-2 (Ikelite 2-pin) are examples of wet connectors. Typically used for low-voltage and signal connections, i.e., telephone, flash triggering, and video.

Whitworth (Whit., BSW). Obsolete British Standard screw thread system based on a 55° pitch angle. The standard camera tripod socket thread was originally 1/4" BSW 20, where the 20 refers to the number of threads per inch (tpi). It so happens that screws and sockets conforming to the the American Unified thread 1/4UNC20 specification (60° pitch angle) will fit with 1/4BSW parts, and so the 1/4UNC thread is now the de-facto standard for tripod attachments.

Wide-angle lens: A wide-angle lens is one which has an angle of view significantly wider than a normal human perspective. Humans actually have a horizontal field of view of about 180°, but can only see clearly in the central field, which corresponds to about 40°. To complicate matters, lens manufacturers like to quote the angle of view corresponding to the diagonal of the film format (because this gives a larger number), and the effective angle changes when a supplementary lens is added. As a rough guide, a `normal perspective' or `standard' lens for use in air has a focal length approximately equal to the width of the film format multiplied by √2 (i.e., W × 1.414). E.g., for the 35mm format (36 × 24mm), the standard lens is 36 × 1.4 = 50mm. For square medium format (57 × 57mm), the standard lens is 57 × 1.4 = 80mm. For a video camera, a typical format might be 6.4 × 4.8mm, giving a normal view when the zoom lens is set to about 9mm. If you use an `air' lens underwater, note that the optical port counts as a supplementary lens, and that a flat port reduces the effective angle of coverage. Consequently, the standard focal length for an underwater 35mm camera (assuming a flat port) is taken to be 35mm (although the logic behind this convention is not strictly correct, since it corresponds to the `normal view' of someone who can see underwater without a diving mask). Any lens with a focal length longer than standard is called a `Portrait' or `Telephoto' lens. A lens with a focal length shorter than standard is a wide-angle lens.

Xacti: Sanyo trade name for combined video and stills cameras.

Xenon, Xe: (pronounced 'kzee-non' or 'kzen-on', the 'k' sound being slight or inaudible) A member of the family of gases known as noble gases (Helium, Neon, Argon, Krypton, Xenon). A photographic flash tube is filled with Xenon at somewhat less than atmospheric pressure (usually between about 0.1 and 0.5 bar). The Xenon gas electric-discharge photographic flash was invented in 1931 by Prof. Harold Edgerton of MIT. Edgerton travelled with Jacques Cousteau on the Calypso expeditions, in which context he was known as 'Papa Flash'.

X-Ray: That part of the electromagnetic spectrum above ultraviolet in frequency (ie., of shorter wavelength than UV, but not as short as Gamma-rays).

X-Sync.: Electronic-flash synchronisation, as opposed to bulb-flash synchronisation. In electronic flash photography, the light discharge is very rapid, so (in the context of a mechanical film camera system) the camera shutter must be fully open before the flash unit is triggered. In cameras with a focal-plane shutter (a shutter which lies right in front of the film) the shutter does not open fully when high shutter speeds are used. Instead, the second-curtain of the shutter is released before the first-curtain has finished travelling, and the film is exposed by a slit passing across it. Consequently, X-synchronisation is not possible above a certain speed. The highest speed at at which electronic flash synchronisation is possible for a given camera is called the 'X-sync. speed'. Synchronisation is, of course, possible at slower speeds, but the effect of ambient light becomes more and more important as the speed is reduced, and at some point, the exposure due to ambient light may exceed that obtained from the flash. In modern camera systems, a 'ready' signal from the flash is used to set the camera to the X-sync. mode if the user tries to select a higher speed. In digital systems, the 'shutter' is virtual, i.e, the exposure is managed by means of a start and a stop signal sent to the sensor, and management of flash synchronisation is done by the system microcontroller and does not usually need to be considered by the user.

Y/C: Luminance and Chrominance, i.e., the 'brightness information' and 'colour information' parts of a composite video signal. The symbol Y was chosen long before the computer age, when TV and printing technologies were unrelated (the potential confusion being that Y also stands for yellow).

YS: Yellow Submarine (we all live in one). Beatles inspired name for Sea & Sea's first ever product (a flash unit). Sea & Sea strobes are still called "YS", even though most of them aren't yellow any more. yellow-sub
YS-mount: The attachment between Sea & Sea, Epoque, Sony and Olympus flash-units and lights and the lighting support bracket (arm). Fixing-bolt thread is M8. Distance between clamping cheeks is 12mm. YS-mount

YUV: Y=Luminance, U=Normalised B-Y, V=Normalised R-Y. The colour space representation used in PAL video and television transmission.

Z: North Americans pronounce the letter "Zee", and certain abbreviations such as E-Z P-Z (easy-peasy) only make sense when you use the American pronunciation.

Z-mount: Mounting attachment on Inon flash units. M6 socket with facility for an anti-rotation pin.


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