Glossary
of photographic & technical terms
# (hash symbol): Abbreviation for 'Number' (North
American), equivalent to 'No.'.
e.g., 'Product #4101' = 'Product No. 4101'.
f: (Greek letter 'phi'), abbreviation
for 'diameter' (used because it consists of a line drawn across
a circle).
 : Female.
 : Male.
¥: Infinity. |
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.
Aberration: 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. More information. See also DC.
Achromat: 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.
Acrylic: Poly methyl methacrylate. A clear plastic with good
optical properties and very high transparency. 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. The
problem with automatic exposure is that it usually relies on
assumptions about the reflectivity of the subject, and it cannot
anticipate the photographers artistic intentions - see TTL metering.
AF: 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. More Information.
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.
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. |
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 M6x1, 13BA fits M1x0.25.
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.
bar: Unit of pressure in the SI (systeme internationale)
metric system. Standard atmospheric pressure is "one thousand
and ten millibars", ie., 1.01 bar. 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. |
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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 (expired). See pixels
article. |
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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. |
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BNC connector: Bayonet Neill-Concelman
(named after the designers, Paul Neill and Carl Concelman). Coaxial
connector used for video, ethernet, and radio frequency signals.
Refs. |
|
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 tend to 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 called a 'video 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 making each of the individual
cells 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. Cheaper video cameras
use a single CCD array with a colour mosaic filter to separate
red, green and blue. Expensive cameras split the light into red,
green, and blue optically, and use 3 CCD arrays.
Cell (electrical): An electrochemical energy source. A
device with a particular voltage difference accross 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 black body is an object which
produces a smooth spectrum when heated (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). More information.
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.
Diopter:: A unit used to express
the power of magnifying glasses. The power of a lens in diopters
is the reciprocal of its focal length in metres, eg., a 2 diopter
lens has a focal length of 0.5m. Screw-in magnifying lenses,
which can be fitted to the front of a camera lens, are sometimes
referred to loosely as 'diopters'. More information.
Dispersion: See article on refraction.
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.
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. More
information.
E/O connector: Electro-Oceanic: A type of wet-connector
with two contacts, in appearance like a rubber jack-plug. See
flash connectors page for more
information.
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
amendment.
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.
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 x
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 is equal to the focal length. As the lens is moved away
from the film, 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 SLR cameras usually have control linkages
or electrical connections passing through them to retain any
automatic functionality of the lens. Extension tubes for the
Nikonos 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. See the FAQ on
lenses (macro lens setups)
for a discussion of the implications of extension tubes with
respect to resolution. See also: macro
lens. |
Fasteners: Generic term or collective
noun for nuts, bolts, screws, washers, and other parts or devices
used in attaching one object to another.
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 perverse, to describe a close-up
lens as a filter, the real reason 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 simply too bright for the camera equipment. 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 occasional perverse decisions
on the part of manufacturers, camera filter screw threads are
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 M58x0.75, but usually abbreviated to M58.
FireWire: See i.Link |
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Fish-eye: 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., objects at the edge of the frame
appear stretched in comparison to objects 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 'fish-eye'
lens. |
Fish-eye distortion of rectangular test card. |
Fish-eye photograph with vanishing point
placed at the centre of the image.
Illustrations ©
Cameras Underwater. |
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 (e.g., the film)
and the middle of the lens, when light from infinity is brought
to a focus. I.e., you can estimate the focal length of a simple
lens by projecting an image of the sun onto a rock, or some other
hear-resistant object, 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. If you focus
an image of the sun onto a piece of paper, the paper will burst
into flames if the image is bright enough). 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, in which case the term
serves a shorthand for the associated angle
of coverage. Unfortunately, the commonly understood relationship
between focal length and angle of coverage breaks down when you
change the optical medium; i.e., lenses of a particular specified
focal length are not as wide-angle underwater as they are in
air. E.g., a 15mm underwater lens has about the same coverage
as a 20mm lens designed for 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). The new Four-Thirds format for digital cameras
has a diagonal measurement of 22.5mm, regardless of the chosen
aspect ratio (i.e., any combination of horizontal and vertical
size may be used provided that the diagonal is always 22.5mm).
Format sizes of the various digital imaging sensors are given
in the pixels 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 http://www.four-thirds.org/en/). So far it
is supported by Olympus, Kodak, Fujifilm, Panasonic, Sanyo, and
Sigma. The specification dictates a minimum image circle diameter
of 22.5mm, so that although the format diagonal must always be
22.5mm, the aspect ratio (width x height) may differ between
cameras. If the 4:3 aspect ratio is used however (as is the intention),
the format will be 18 x 13.5mm. An important feature of the specification
is that the throat diameter of the lens mount is much larger
than the format diagonal. This allows the use of telecentric
lenses (lenses from which light emerges almost parallel to the
axis), thus solving a problem of digital image sensors, which
is that there is a fall-off in sensitivity and degradation of
the ability to separate colours for light rays arriving at an
oblique angle.
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 = pr²), 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 ( Ö2 = 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 slowly being 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, the flash fires, and the shutter closes,
but because the flash duration is very short, the shutter has
no control over the amount of light falling on the film. Consequently,
the level of film 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 film speed and distance (in air) is
obtained thus:
normally, this equation is used to produce a guide table, which
is attached to the flash. |
|
HAD: Hole Accumulation Diode. Diode structure
used in some types of CCD image sensor. |
Handy: Quasi-English term (malapropism) used in Germany
to mean 'Mobile Telephone', and used in other countries to mean
'hand-held'. In normal English idiom, handy simply means 'useful'.
Other usages appear bizarre to native English speakers.
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.
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". |
Nikon
hot shoe |
Olympus
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. |
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 19th century were important to the development
of wireless telegraphy (sometimes also known as 'radio'). Due
to minor diplomatic differences between Britain and Germany during
the 1930s and 40s, the unit was not properly adopted by the Anglophone
world until the 1960s, and some people still persist in using
c/s. |
ICS: Ikelite connector system. See also:
flash connectors page.
ICS-2: A 2-pin wet connector used with early Ikelite manual
flash units.
ICS-4: An obsolete 4-pin O-ring sealed connector used
with early Ikelite automatic flash units.
ICS-5: A 5-pin O-ring sealed connector used mainly with
Ikelite TTL flash units and video lights.
ICS-10: A 10-pin O-ring sealed connecter used with scientific
and technical instrumentation.
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 is 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 Imperial measurement systems were
finally made compatible in the 1960s by a small adjustments to
the inch and the metre 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 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.
See IP ratings article.
IR, Infrared: Beneath red. That part of the electromagnetic
spectrum (light) which lies just below red in frequency (ie.,
of longer wavelength than red). See also electromagnetic radiation.
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.
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 with a magnifying glass. 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., M6x1
= 6mm diameter, 1mm pitch; M8x1.25 = 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 M14x1, specifying it as M14 will not do, because the coarsest
14mm thread is M14x2. Camera filter threads are an exception
to the 'always state the pitch unless it's the coarsest one'
rule however because, apart from occasional perversity on the
part 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 M6x1 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. Modern
developments in Ni-Cd technology have largely eliminated the
memory effect, but the myth of its existence persists. In fact,
more damage is done to cells by people trying to avoid the effect
than by people who ignore it altogether; the reason being that
batteries are likely to fail permanently if discharged significantly
below 1V per cell, which is exactly what happens if you leave
equipment on until the batteries are completely flat - Don't
do it!. It is also inadvisable to leave Ni-Cds unused for
long periods, because they will self-discharge slowly and eventually
drop below the point of no return, so take them out occasionally
and recharge them. If you are superstitious about the memory
effect, use an electronic battery discharger, i.e., a device
which takes the battery down to 1V per cell and then stops drawing
current.
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.
Ni-Cd, "Nicad": Nickel-Cadmium rechargeable
battery. See also memory
effect.
Nikonos: Underwater camera system, manufactured by Nikon;
developed from the 'Calypsophot' camera designed by the Belgian
engineer Jean DeWouters for the Calypso expeditions of Jacques
Cousteau. See the Nikonos
article for more background.
Nikonos grease: Petroleum gel (also marketed as 'Vaseline').
Suitable lubricant for NBR (black nitrile) O-rings, but somewhat
inferior to silicone grease.
Nikonos connector: See flash
connectors page.
Nikonos V: Pronounced "Nikonos five", or "Nik
five", not "Nikonos vee".
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. For more
information, see the TV
standards article. |
Ohm, W: 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.
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. For a
less terse discussion of how O-ring seals work, see the starting out section. For more information
on O-ring care, see the FAQ on maintenance.
See also Shore hardness.
OS: Operating System, i.e., Windows, Mac OS, Unix, Gnu/Linux,
etc.
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.
For more information, see the TV
standards article.
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.
PC: Polycarbonate.
PCB: Printed circuit board.
PCM: Pulse-code modulation. Digital audio recording mode
used by tape recorders.
Pel: 'Picture element'. Now largely obsolete. The preferred
alternative 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.
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 (see the pixels
article for more information).
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.
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 x 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 x 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
from one medium into another, e.g., from air to water, or from
air to glass. More information.
Refractive Index: See article on refraction.
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. See pixels and
resolution article.
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 squared.
Of an alternating voltage or current: the equivalent steady (direct)
voltage or current which will have the same heating effect.
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. See flash
connectors page for more information.
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. For more information, see the TV
standards article.
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.
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.
Sn: Tin (stannum).
Snell's Law, Snell's Window: see article on refraction.
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".
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. In the author's opinion, it is best to get used to the
American term; since if you are travelling in a foreign country,
and 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.
If you don't like the term 'strobe', try 'speedlight'.
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.
Substrobe (SS-): Ikelite's name for an underwater flash
unit.
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. |
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.
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: Through The Lens
metering. The basic method for determining photographic exposure
is to measure the intensity of the ambient lighting and choose
camera settings accordingly. The problem however, is that filters,
lens extension, and other factors must be taken into account,
and the way around this problem is to examine the light which
comes in 'through the lens'. One obvious advantage of this procedure,
is that the light reading so obtained can be used to set-up the
camera automatically; but a major disadvantage is that the reading
is dependent on the reflectivity of the subject. TTL metering
systems assume that the subject will reflect 18% of the light
falling on it. Many subjects do reflect about this much, and
are said to be of 'average reflectance'; but many do not and
the resulting exposure can be seriously incorrect. The non-average
problem can be solved in several ways, ie.,
(1) give-up and use a proper light-meter,
(2) use films with a wide exposure latitude,
(3) develop an eye for what is average and what is not, and perform
exposure-bracketing when not sure,
(4) get a camera with matrix TTL metering.
In addressing the second of these points, film manufacturers
developed amateur films with enormous exposure latitude. A typical
amateur negative film will produce acceptable prints with exposures
which vary between +3 and -2 stops from the correct value. Transparency
(slide) films however, and some professional negative films,
must still be exposed correctly; and proper light metering, or
exposure bracketing, is the norm when using these materials.
Expensive cameras with matrix TTL metering assess the brightness
of the scene with an array (matrix) of light sensitive cells,
rather than a single cell, and can usually (but not always) expose
slide film correctly without help from the user. In digital cameras,
the metering matrix can be derived by averaging the information
from the imaging device (CCD). It is however, not wise to assume
that digital cameras will always make good exposures, because
the camera must still make a decision regarding how to fit the
contrast range of the scene to the contrast range of the sensor
and, especially if the former exceeds the latter, the automatic
choice may not always be acceptable.
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 largely 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
spectrum (see electromagnetic
radiation) which 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.
UVA: That part of the ultraviolet spectrum nearest to
visible light, ie., long wavelength UV.
UVB: Between UVA and UVC.
UVC: That part of the ultraviolet spectrum nearest to
X-rays, ie., short wavelength UV.
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. |
UR/Pro UWCC
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 verb; to 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 strobe lighting tray; "Lens-holder
W" = dual lens holder.
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
x Voltage (P = IV), eg., a 12V light-bulb which consumes 60W
must draw a current of 5A.
Wavelength: see Electromagnetic
radiation.
Wet Connector: A connector which can be un-plugged and
re-connected underwater. Sealock, E/O (Electro-Oceanic), and
ICS-2 (Ikelite connector system, 2-pins) are examples of wet
connectors. See flash connectors
page for more information.
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 Ö2 (i.e.,
W x 1.414). E.g., for the 35mm format (36 x 24mm), the standard
lens is 36 x 1.4 = 50mm. For square medium format (57 x 57mm),
the standard lens is 57 x 1.4 = 80mm. For a video camera, a typical
format might be 6.4 x 4.8mm, giving a normal view when the zoom | |
