by Brian Pitkin

Reproduced from in focus 22 (June 1987)

A resumé of the BASIC COURSE talk given at the May meeting

The greatest obstacle you will have to overcome when taking underwater photographs Is the water. Not only Is it trying to get into your camera and flashgun, it filters light and distorts the image. The greater your depth, the less light will penetrate and the less colour will be apparent. This BASIC COURSE talk will briefly discuss ARTIFICIAL LIGHT and its use to help overcome the filtering effect of water on natural or available light.

Although It is possible to take excellent underwater photographs using natural light. unless you stay extremely shallow your results will lack colour and unless you use fast film the depth of field will not be very great. The slower your film, the greater the detail recorded, but additional light will be required.

Artificial light can be provided In one of two ways, either by a continuous light source such as an underwater cine light or by a flash of light from a flash gun. Both sources have advantages and disadvantages.

CONTINUOUS LIGHT

Continuous light has the advantage that you can see what you are Illuminating, you can use a light meter to determine the correct exposure and usually the area of light is diffuse. Unfortunately the available cine lights are balanced for tungsten and a blue filter Is essential or else a reddish light results. You also need a slower shutter speed or a larger aperture because the available cine lights are not that powerful. In addition cine lights are not suitable for light sensitive animals and can be quite bulky.

FLASH

Flashguns provide a rapid pulse of very bright light which freezes motion. You cannot see the effect when you take the picture and unless you are using a dedicated auto flash unit. you need to calculate the exposure beforehand. Unlike cine lights, they can be used for lightsensitive animals and are quite compact for their output.

Two types of flash are available, either Bulb or Electronic. Early underwater photographers had only bulb flash and although these are occasionally still used have largely been superceded by electronic flash. Fortunately land flash bulbs are water and pressure proof so they can be used underwater. The flash contacts are an the outside of the bulb and consequently open to the water. Having to ensure that the contacts were perfectly clean and the bulbs correctly positioned led to a high failure rate. Coupled with this the expense - 36 bulbs cost more than a roll of film - and the need to change bulbs after every exposure led to their demise in the face of the cheaper and more convenient electronic flashgun, although they can provide a quality and power unmatched by any electronic flash of comparable price.

The majority of underwater photographs are taken with additional lighting, electronic flash being the mat widely used form and the simplest method of replacing light and therefore colour.

As with most modern electronic Items, flashguns vary from the very basic to the extremely complex, but they all work an the saw principle - they convert a small voltage Into a auch larger one, A battery or batteries charge a capacitor to a high voltage. Once sufficiently charged a 'ready light' cones on. As the shutter release is pressed a charge is sent across a gas filled flash tube. The sudden high voltage ignites the gas producing a bright flash for a split second.

TYPES OF ELECTRONIC FLASH

Of the range of underwater photography equipment, the biggest choice must be In the field of electronic flashguns. Yhether amphibious or land flashguns In a housing they way be categorised by angle of coverage.

COVERAGE UP TO 60°

The majority of flashguns fall in this category. They are simple electronically and use a conventional reflector. They produce a very direct beam of light which because it is from a small point source giver. fairly harsh shadows and at the edge of the light spread there is a marked cut off between light and dark. To maintain the land angle of land flashguns, most units have a dome in front of the reflector.

COVERAGE OVER 60°

These are more specialised and there is less choice. If using a wide-angle lens it is usually essential that the flashgun will cover the same angle. The reflector is designed to be inefficient, spreading the light over a wide area which in turn decreases the power of the light. In order to restore the power the electronics are improved. The resulting unit is larger than a land flash but more versatile.

Of course the angle of any flashgun may be increased by the use of a diffuser but this will decrease the power of the flash. It has the advantage when used for macro and standard lenses in producing a more diffuse light and hence shadows are riot as harsh.

AMPHIBIOUS VERSUS HOUSED

There is a greater choice of land flashguns and provided a suitable housing is available these will perform equally well underwater. Land flashguns are cheaper and should the housing flood can be replaced more cheaply than the insides of an underwater flash. Most housings however have a flat port. reducing the land angle and you must either diffuse the land flash or consider an amphibious flashgun with a dome correction port.

Electronic flashguns range from simple single power units to models with dual or multiple power slave mode and modelling lights. Sow amphibious units incorporate a leak detection system. Dual or multiple models give greater versatility. Models incorporating a slave setting cat be fired remotely by the flash from the master or primary flashgun connected to your camera. In this way more than one flash can be used at a time.

Alternatively two or more flashguns my be fired by the same camera using adaptors but this increases the chances of one or more flashes failing to fire.

POWER SOURCE

Electronic flashguns are powered by batteries. These my be either disposable alkaline or rechargeable ni-cad batteries. Disposable batteries will give more flashes but the cycle time between flashes will gradually increase as the batteries weaken. li-cad batteries will give fewer flashes but the cycle time will vary very little until the batteries are exhausted. It Is important when using either type to form the capacitor by firing the flashgun several times prior to use, preferably with an older set of batteries, as initially the output will be less than optimum. If using ni-cad batteries it is important to train them by fully charging, fully discharging and recharging them again, as they can get used to only being partially discharged and partially recharged.

TEST EXPOSURES

Most flashguns are provided with an exposure guide. This is however only a guide and since some manufacturers my exaggerate their products output you should run a test. Using slide film. which is less forgiving when It cows to under- or over-exposure, take a series of pictures of a colour test card or similar colourful subject at all available apertures and at a range of lens-to subject distances. A swimming pool is Ideal for this, but remember In poorer visibility it my be necessary to increase aperture (decrease f-stop number) slightly. Nake up a test chart before you run the test and follow It religiously. A test requires very little effort and will give you a very good guide as to your flashgun's capabilities. Once your film Is processed study It carefully comparing the results with your test chart. Decide which apertures give the best results for given distances and use these to make up a working guide for your flashgun.

Having brelfly discussed the reasons for using artificial light and the various units available that can be employed to provide it let us now consider some aspects of using artificial light.

BACKSCATTER

As soon as you use an artificial light in less than gin-clear water there is a problem of backscatter. The particles of sediment and plankton in the water which reduce visibility will reflect light from the flashgun back Into the camera lens, degrading the image and at its extreme filling the resulting photograph with "snow". One way to Improve this situation Is to move the flashgun away from the camera lens so that you illuminate fewer of the particles between the subject and the lens. Alternatively move closer to your subject to produce the same effect.

DIRECTION OF ILLUMINATION

To provide a feeling of naturalness and reduce backscatter the subject should be illuminated from above and to one side. Either hand hold the flashgun or have It mounted on a long versatile arm. A flashgun attached to the camera near the lens is no good, except for extreme close-up. The best general compromise is to have the flashgun above and to the left at 451 to the lens-subject axis. Illuminating the subject from beneath or behind can give a creative effect. Side lighting increases shadow length and can be good for certain shots.

VARIABLE POWER

If the flashgun provides more light than is available naturally then the background will be black. Although some people do not like black backgrounds, they do help to draw the eye to the subject. At night all backgrounds will be black. If you do not want black backgrounds during daylight either balance the flash output with the available light by reducing the power of the flashgun or by moving the flashgun further from the subject.

SLAVE FLASH

A slave gun can be used some distance from the camera to provide a second source (and direction) of illumination or to simulate a 'torch' held by a diver. If used remotely you my need to fix the gun In position as most are positively bouyant. If using two flashguns of equal power at similar distances from the subject and camera the amount of light is doubled and the exposure increases by one stop.

SPECIAL EFFECTS

As well as back lighting or side lighting your subject It is possible, though not necessarily desirable, to change the colours by filtering the light from the flashgun. Thus if you use a red filter on the flashgun, your subject will have a red caste. Of course if you use filters you will have to recalculate the correct exposure.
Yhatever artificial light source you use you will need to run a test film, experiment and keep experimenting. Start with macroor close up and stay with it until you have mastered exposure and then, and only then, move on to larger subjects and balanced light - the subject of June's BASIC COURSE.