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Artificial Lighting for Underwater
Photography
By David Knight
Apart from the need to make the equipment waterproof, the major
differences between photography in water and photography in air
are due to the fact that that water absorbs and scatters light.
Human vision adapts automatically to this change of environment,
but camera systems require help and persuasion if they are to
produce good results. The specific problems which have to be addressed
in this respect are as follows:
Water
absorbs light across the whole of the visible spectrum. Water
may look clear in a drinking glass, but light absorption over
distances of several metres is significant. Hence the underwater
environment is dark, artificial lighting is often needed, and
flash and video lights are not as effective as they would be in
air.
Water
absorbs light at the red end of the spectrum more strongly than
it absorbs light at the blue end. Water may look colourless in
a glass, but a glance at a swimming pool shows that it imparts
a cyan cast to light which travels
any appreciable distance through it.
Water
usually has particles suspended in it; from small solid particles,
organisms, and gas bubbles, which give the impression of fog;
to large fluffy particles which give the impression of a blizzard.
The brain processes visual information to enable us to see through
partial obstructions, but the processing relies on stereo vision
and movement and is much less effective when we look at two-dimensional
still images.
These additional factors change the rules considerably in relation
to lighting intensity requirements, working range, white balance,
and lighting position. It might also be said that 'lighting technique'
not only encompasses the use and positioning of lights, but the
image adjustments which can be carried out after the event in
order to restore colour balance and contrast. With the latter
in mind, those interested in obtaining professional quality results
should use cameras (or film scanners) which can provide output
in a 48 bits-per-pixel 48bpp) format (e.g., RAW), the point being
that an image file with 4096 brightness levels for each primary
colour (rather than the usual 256) can be subjected to large adjustments
without significant quality loss.
Reduced Guide Number
Flash illumination underwater no longer follows the inverse-square
law on which 'in air' exposure guide tables are based. At a flash
to subject distance of about 1 metre, the guide number is reduced
to about a third of its 'in air' value. At greater distances,
the guide number is reduced even further, and only when the light
source is very close to the subject does the guide number begin
to approach its 'in air' value. Nowadays, we don't worry about
guide numbers too much, taking care of exposure by looking at
the image histogram or by using TTL flash, but it means that a
large flash unit is needed if an appreciable range is to be obtained.
Colour Cast
What is meant by 'appreciable range' may also come as a surprise,
the issue being that clear water acts as a strong cyan-blue filter.
We need to remember here that white light is a mixture of all
the colours of the spectrum. The problem is that the different
colours are not absorbed equally by water. Red is absorbed particularly
strongly, leaving pictures taken at intermediate range with a
cast of the complementary colour, i.e.cyan.
For very long range (10m or more), both red and green are absorbed
and the only colour substantially recorded is blue. In the latter
case, little or no colour information is present in the picture,
and the best presentation may well be obtained when the colour
saturation is turned down to zero and the picture is reproduced
in black and white.
In clear water (free from green algae or brown mud) the red component
of white light is reduced relative to the blue component by about
1 EV (i.e. halved) for every 2.5m of light path. Since the light
has to travel from the flash unit to the subject, and then from
the subject to the camera; if the flash is aimed from the vicinity
of the camera, a photograph taken at a distance of 1.25m has its
red component reduced by the equivalent of 1 f-stop. In the old
days of direct-projection transparency film, this meant that the
maximum flash illumination range was in the order of 1 to 2m,
and the strong cyan cast was taken to be a fact of life. Compensation
was possible at the printing stage, and nowadays, we can adjust
the image data to pull out the cast; but electronic noise becomes
a problem if the gain of the red channel is adjusted too far,
and so the trick in producing clear colourful pictures still lies
in working at short range.
Backscatter
A particular problem arises when using flash in cloudy water,
and is exactly analogous to that of trying to use full-beam headlights
when driving in fog. Light is scattered back by the suspended
particles, and the subject can sometimes hardly be seen at all.
Natural water is always a little cloudy, and swimming pools are
only clear when properly maintained*; so if high clarity is required
it is important to check the state of the water and keep extraneous
swimmers and divers away from the location for a few hours so
that particles can settle. For poor visibility which cannot be
avoided, the problem is reduced by aiming the camera and light
source from different directions (the point being to light only
the subject and not the suspended particles between the subject
in the camera); and by working at very short range (i.e., by using
wide-angle and macro lenses). Remedial action at the post-processing
stage involves adjusting the image black-point (Photoshop: levels)
to crush-out the general background fog and increase contrast;
and using the cloning stamp to remove any distracting white spots.
* Sand filtration alone is not enough. Perform
a good backwash a few days before a shoot and add some flocculating
agent (e.g. Aluminium Sulphate) to the input side of the filter.
 BACKSCATTER
Above: Picture spoilt by silt kicked-up by divers who had visited
the site immediately prior to the shot. The suspended particles
are barely noticeable to the diver, but firing a flash lights
them up.
Right: the same image retouched using the Photoshop cloning stamp,
with the black-point adjusted slightly to disguise the general
fogginess. |
 |
Backscatter, even in water which appears to be clear to the naked
eye, is a severe problem when using the internal flash of a compact
camera. It is for this reason that an external flash unit, which
can be moved away from the lens as necessary, is recommended whenever
artificial lighting is to be used.
Lighting position
|
In the dim and distant past there was something called 'standard
lighting', which involved having the flash unit mounted on an
arm and tray and positioned above and to the left of the camera.
Whoever coined the term it is not politic to recall, because
this lighting position is mostly a bad idea. It might better
be called the 'apathetic' or 'not very good at controlling my
buoyancy' lighting position, because the immediate reaction to
any picture so produced is "It looks like it was lit from
the left". The point is that it was not intended that the
flash unit should be routinely fired from that position, the
clue being in the fact that there is a coiled cable connecting
it to the camera. The attachment of the arm to the tray allows
the diver to get into the water with a hand free to trim buoyancy.
After that, the lighting arm is released and the flash is positioned
by hand, generally in anything but the so-called 'standard' lighting
position. |
"STANDARD" LIGHTING ?
|
Nikonos V camera shown with SB105 and SB104 speedlights.
The flash arm can be detached from the camera tray by loosening
a nut or by pressing a button. The coiled cable can be made to
extend to about 2m. |
|
Ball arms
For a small compact camera, a standard arm and tray is adequate,
but not necessarily optimal. For something bulky like an SLR housing
moreover, the camera needs to be controlled using both hands,
and there is no spare hand to hold the flash unit. The solution
is the fully-articulated ball-joint arm, which permits nearly
all of the lighting positions which are possible by hand-holding,
and will keep the flash unit rigidly positioned relative to the
camera.
|
Ball arms were invented a long time ago, but manufacturers took
a while to perfect their designs. There were many products which
couldn't support the weight of a medium-sized flash unit in air
and tended to incur damage to the balls if the clamps were done
up tightly. One company got it right first off however, and that
was the diving equipment manufacturer Oceanic. The trick was
to use a ball of 1" (25.4mm) diameter, and some old-timers
still call this ball size the 'Oceanic ball' to distinguish it
from the smaller sizes which didn't work properly. Oceanic pulled
out of the underwater photography market in the 1980s, but the
design legacy remains in that the 1" ball, or its metric
equivalent 25mm, has been widely adopted. In particular, 1"
ball joint arm components are made by Ultralight, TLC and Ikelite,
and 25mm components are made by Inon, 10bar, and others; parts
from the inch and metric ranges being compatible by virtue of
a tolerance in the clamping system of about ±2mm difference
in ball sizes. |
Ultralight ball joint |
Ikelite, being a manufacturer of large flash units, also makes
a 1¼" (31.75mm) ball-arm system, which is capable
of holding-up lighting-heads weighing more than 3Kg in air. There
is also an Ikelite adapter (#0466.51) which allows mixing of 1"
and 1¼" parts. The Ultralight 1" system (for
example) can hold-up 3Kg in air, but the clamps have to be done-up
tightly.
Quick-release
Assembly and disassembly of a ball joint is a somewhat fiddly
procedure, and there are occasions when a ball joint arm does
not extend far enough or does not permit a particular lighting
position. Hence, to allow maximum versatility, and to facilitate
dismantling of the equipment for transport or to make it easier
to change memory cards and batteries, it is always desirable
to have some means by which the arm can be detached quickly from
its base. Once again we call upon the legacy of Oceanic, with
its novel concept of allowing people who actually used the equipment
to be involved in the design process; their offering, the Oceanic
Shoe (universal shoe or T-base), having been copied by numerous
manufacturers and remaining a perfectly good choice. |
Oceanic Shoe (Ultralight) |
An alternative to the Oceanic shoe is the TLC dovetail base, which
is equally good; but better still are the Ikelite Quick-Grip and
the Ultralight Quick-release handles, both of which allow arm
detachment at the press of a button.
Forward Lighting
If there were to be a proper 'standard' (i.e., 'quite often gives
a good result') lighting position, it would be with the flash
unit mounted directly above the camera lens and facing
forward. The reason is that a picture which gets progressively
darker as the eye runs from the top to the bottom is psychologically
neutral with regard to lighting - i.e., the viewer is unaware
of the lighting and sees the subject first.
FORWARD FACING: A sensible requirement for
an underwater camera system is that it should be able to provide
light from directly above the lens without the user having to
hold the flash unit by hand.
Note that, for the compact camera system on the left, the
flash window is vertical when the arm is bent inwards, i.e., the
flash unit has been designed so that the window is horizontal
when the flash is in the off-side "standard" lighting
position. Having the window the wrong way around reduces the coverage
slightly, but the problem is readily solved by fitting the flash
unit with a diffuser.
The disadvantage of the hot-shoe flash system on the right
is that it can only do forward lighting.
The forward lighting position is ideal for medium range working
(0.5 - 3m) in conditions of fair to good visibility. For this
purpose, the camera is best fitted with a wide-angle lens (or
converter) having a field of view (FOV) in the 63 – 100°
range (35mm equivalent focal length 35 - 18mm when using a dome
port), the point being to keep the working range fairly short.
Unless post-processing correction for chromatic aberration is
to be carried out, lenses of greater than about 63° FOV in
air (35mm equiv: 35mm) are not recommended when using a
flat lens-port. Hence, choice of port is a factor in determining
how close the camera can be for a given size of subject.
In practice, of course, the flash unit should preferably be tilted
down to aim it directly at the subject. The more expensive underwater
flash units usually have a circular flash tube with a spot beam-pattern
targeting lamp mounted in the middle to facilitate accurate aiming.
The targeting lamp has the additional benefit that it illuminates
the subject so that the camera auto-focusing system can operate
in dark conditions. When using very low levels of flash illumination,
it is important that the targeting lamp should be switched off
during the exposure, otherwise its illumination spot will be seen
in the photograph. LED targeting lamps can be switched off automatically
during the flash exposure, but filament lamps, being slow to respond,
have to be switched off manually.
 |
TARGETING LAMP.
Shown is an Ikelite DS-160 Substrobe, which has an LED targeting
lamp. When the flash is triggered, the lamp is automatically
extinguished for the duration of the flash exposure. |
The forward lighting position is also the natural choice for mixed
lighting (i.e., balanced natural and artificial light), sometimes
called 'fill-in flash'. Some cameras are able to compute the intensity
for for fill-in flash when a TTL flash system is used. When using
a manual flash system, the trick is to choose a shutter speed
which allows flash synchronisation, then meter for a natural light
exposure but choose an aperture setting which is somewhat smaller
than the metered value (between ½ and 2 stops usually).
The flash output is then adjusted to make up the exposure shortfall.
White balance is normally set for flash or normal daylight in
this case, and after post-processing adjustment to set the colour
balance for the principal subject, the natural light contribution
will retain a cyan or blue colour cast.
MIXED LIGHTING: Here the colour balance has
been adjusted in post processing to suit the foreground and the
black-point has been adjusted to disguise the fog. Note how the
colour cast changes from cyan in the mid distance to deep blue at infinity.
Bounce Lighting
With a forward-facing flash unit, bounce lighting is possible
in clear shallow water because any light-ray striking a water-air
boundary at an angle of more than 48° from the perpendicular
is reflected back into the water. This is the phenomenon of total
internal reflection, the critical angle of 48°
being dictated by the refractive index (n) of the water. Bounce
lighting will be inefficient if the surface is foamy or broken-up
by waves; but if the surface is reasonably flat, virtually all
of the light hitting the surface at greater than the critical
angle will be returned.
For bounce lighting, the subject should be further away than for
normal forward lighting, and consequently a lens of somewhat longer
focal length than usual (e.g., 50mm) should be used. The point
is that if the subject is too close, most of the reflected light
will fall behind it. Due to the long light-paths involved, considerable
adjustment of colour balance will be required in post processing
to restore the reds and yellows in the image, and so bounce lighting
is best accomplished using systems which can output 48bpp image
data.
BOUNCE LIGHTING: Light heading straight for
the surface escapes, but light rays which are not perpendicular
to the surface will undergo total internal reflection if
the critical angle is exceeded. The critical angle for fresh water
is 48.6°. For sea water, the critical angle is 48.3° (i.e.,
it is about 48° in either case).
MIRROR: The water-air boundary acts as a highly
efficient mirror for light rays arriving at a shallow angle. This
applies to light from the flash and also to light returning from
the subject.
Top Lighting
Like forward lighting, top lighting produces images which become
progressively darker from top to bottom. The difference however,
is that top lighting produces much stronger shadows. It follows,
that top lighting and forward lighting are not two distinct techniques,
but rather the two extremes of basic lighting technique. If the
flash is fired straight at the subject, a very flat image results.
If the flash is fired from above, the shadows bring out the three-dimensional
nature of the subject but can be distracting. Hence, it is usually
sensible to choose an intermediate direction which will allow
some light into the shadow regions.
One of the advantages of top lighting however, is that it greatly
reduces the extent to which suspended particles in the water between
the subject and the camera are illuminated. As can be seen from
the diagram below also, there is a tendency for such particles
to be illuminated from behind, so reducing their propensity to
scatter light into the camera lens. Top lighting is therefore
often a good choice in conditions of moderate to poor visibility.
In the diagram above, a light cone with an angle of 100° is
depicted to be emanating from the flash unit, this being the stated
coverage figure for the model shown (Ikelite DS-125) when fitted
with its supplied diffuser. In reality however, flash units do
not have a sharp cut-off at the edge of the useful illumination
field, which means that some of the light from the flash may enter
the camera lens. Lines indicating the field of view of the camera
show that the flash has been positioned so that it cannot be seen
in the photograph, but direct light from the flash can still pass
through the lens and may be reflected from internal structures
and boundaries to cause flare. One particularly annoying source
of flare is the white letters and numbers which some manufacturers
like to put on the front of the lens barrel. These do not matter
when using the lens out of water, but are sometimes visible as
reflections from the lens port. Such markings should be inked
out using a fine-tipped alcohol-based felt-tip pen. Other sources
of flare can be controlled by judicious aiming of the flash so
that no part of the flash tube, reflector, or diffuser can emit
light in the direction of the lens.
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FLARE:
In this case top lighting was used and particles suspended in
the water close to the lens were very strongly illuminated. The
offending particles are either outside the field of view or completely
out of focus, and so would be invisible were it not for their
extreme brightness which causes them to appear as iris-shaped
flare spots in the picture. |
Diffuser
A diffuser increases the angle of coverage of a flash unit and
also the effective area of the light source. Hence a diffuser
provides a more even field of illumination than can be achieved
with a bare tube and reflector, and provides an additional method
for softening shadows. In general, a diffuser should be fitted
unless there is a specific reason for not doing so.
Most modern underwater flash units either have a diffuser included
in the kit or available as an optional accessory. Such diffusers
often have holes in them, for spotting lamps, slave sensors, and
auto sensors. If a proper diffuser is unavailable, it is possible
to improvise one using a piece of thin white cloth with a rubber
band to keep it in place; but if the flash unit has an auto sensor,
the sensor must not be covered.
HARD SHADOWS: Top lighting without diffuser.
SOFT SHADOWS: Top lighting with diffuser.
There are three reasons why a diffuser might be omitted:
Strong
shadows may be wanted for effect.
A diffuser
reduces the intensity of the flash by about 1EV (i.e., 50%). Hence,
with all other variables held constant, removing the diffuser
allows the lens aperture to be reduced by one f-stop. This may
be necessary if depth-of-field is at a premium.
If
top lighting is used to overcome poor visibility, narrowing the
flash beam-angle reduces the extent to which particles between
the subject and the lens are illuminated.
|
Some flash units, of course, have a very wide beam angle even
when a diffuser is not fitted, in which case it is possible to
improvise a device known as a 'snoot'; i.e., a short length of
drain-pipe, car-tyre inner-tube, or a cut-off plastic flower-pot,
which can be fitted to the front of the flash unit. Since a snoot
will add its colour to the emerging light, it should either be
made of black material or should be painted black on the inside.
The length of the snoot dictates the extent to which the beam
is narrowed. |
|
SNOOT: Sea & Sea 'Spot Adapter' for the
YS60 and YS-90. The removable 'honeycomb' front section gives
a near parallel beam. |
|
NARROW BEAM: Top lighting with reduced beam-angle
helps to overcome poor visibility. Here there is no ambient light
because the photograph was taken at night. The SLR housing was
placed on the sand in front of the octopus and the flash unit
was held by hand. The light cone from the flash can be seen clearly
because there is a great deal of suspended matter. Below shows
the same photograph after cropping and retouching.
Macro photography
For macro photography, the same compromise between top-lighting
and forward lighting as is used for general photography is appropriate,
except that the flash unit should be placed close to the subject.
The reason for the proximity is that the lens should be used
at small aperture (large f-number) in order to give good depth
of field, and the flash must be correspondingly bright. It may
be necessary to switch to manual exposure mode in order to force
the camera to use a small aperture, especially when using a compact
camera with an external macro lens (an auto only camera is not
ideal). In the absence of a TTL flash system, exposure is easily
controlled by moving the flash unit closer to or further away
from the subject, or by varying the flash output. |
 |
HIGH CONTRAST: Macro photograph with top lighting
(gills of nudibranch Hypselodoris elegans, about 20mm across.
50mm macro lens at f/22).
LOW CONTRAST: Macro photograph with the flash
unit above and to the right, pointing at about 45° to the
lens axis to make light penetrate between the spines of the nudibranch.
|
A fully articulated lighting arm allows a variety of lighting
positions suitable for macro photography, but can be cumbersome
if macro photography is to be the principal activity. A more
compact arrangement is possible if the camera housing has an
accessory shoe or some other arm base directly above the lens;
a short arm fixed directly to the housing being known as a 'macro
mount'. A macro mount generally enforces a lighting direction
intermediate between top and forward, and is suitable for the
majority of macro photographs. If the enforced direction should
prove to be unsuitable, an accessory shoe fitting can be released
simply by loosening a knurled locking nut. Macro mounts are available
in various lengths (and can be made up from arm components),
and when reasonably long are also suitable for general-purpose
forward lighting in conditions of fair to good visibility. |
MACRO MOUNT |
Dual flash
Some photographers prefer to use a dual flash system. Some
points on this subject worth noting are as follows:
If
both flash units are aimed at different points, the field of illumination
is increased. Given that most cameras have a rectangular format,
spreading the light into an oval pattern can help to reduce the
fall-off of light intensity which sometimes occurs at the edges
of the field when using wide-angle lenses.
Experimentation
While it is important to be familiar with basic lighting technique
and the reasoning behind it, it is not always necessary to adhere
to it. There is often considerable merit in unusual positioning
of flash units or unusual lighting directions. With this in mind,
note that the diver's buddy can be employed to hold the flash
unit much further from the camera than can be achieved by the
photographer alone. Some manufacturers (e.g., Ikelite) make flash
extension cables, one of which, for example, can be used to fire
a flash unit placed within a compartment of a shipwreck while
the camera looks in from outside. Separate slave flash units can
also be placed at some distance from the camera, to be triggered
when the flash attached to the camera is fired.
D. W Knight, July 2006 - Oct 2006
