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 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 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 before or 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 which can provide output in a 48 bits-per-pixel (48bpp, 16 bits per colour channel) file 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 powerful light source is needed if an appreciable range is to be obtained.
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 lighting unit to the subject, and then from the subject to the camera; if a light source 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 can be 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.
A particular problem arises when using artificial light 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 and 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.
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 lighting unit, which can be moved away from the lens as necessary, is recommended whenever artificial lighting is to be used.
In recent years, the sensitivity of digital cameras has improved to the point where it is possible to obtain good photographic exposure using a video light. This is particularly the case for cameras having a fairly large sensor area, i.e., Four-Thirds, APS-C and full frame 35mm formats. It is worth noting therefore, that it is easier to make the lighting adjustments required to minimise backscatter when using a video light (as opposed to flash), for the simple reason that the setting-up of the photograph is done with the light source which will be used when taking the photograph. All of the discussion below relating to flash lighting position and direction applies equally to video lights.
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.
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 lighting 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 light source 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.
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.
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.
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 LIGHTING: 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 lighting 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.
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.
With a forward-facing light source, 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 EFFECT: 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.
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-160) 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.
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.
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 beam-angle reduces the extent to which particles between the subject and the lens are illuminated.
|Some light sources, of course, have a very wide beam angle even when a diffuser is not fitted, in which case a device known as a 'snoot' or 'spot adapter' can be used to narrow the field. Snoots are available commercially; but can also be made from pieces of drain-pipe, car-tyre inner-tubes, or cut-off plastic flower-pots. 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' or 'spot adapter'
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.
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 light 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 light source closer to or further away from the subject, or by varying the light 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 two mating nudibranchs.
|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.||
Some photographers prefer to use a dual flash system. Some points on this subject worth noting are as follows:
ikelite SLR housing with dual TTL flash
| One flash tends to fill-in the shadows left by the other. This means that shadow regions are better illuminated than when using a single flash, and the resulting photographs are generally improved.
Dual flash can produce very odd-looking shadows, especially when used at short range. Use of diffusers is advisable in most circumstances, and both flash units still need to be angled judiciously for best effect.
If both flash units are aimed at the same point, the level of illumination is doubled (assuming that both give the same output). With all other variables held constant, users of dual flash can work at smaller apertures than users of single flash and thereby achieve greater depth of field.
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.
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 lights or in unusual lighting directions. With this in mind, note that the diver's buddy can be employed to hold a 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. Note that underwater tripods are just as useful for mounting lights as they are for mounting cameras.
D. W Knight, July 2006 - Oct 2006. Updated Aug. 2011
© Cameras Underwater Ltd. 2006, 2011, 2012.
David Knight asserts the right to be recognised as the author of this work.