Many photographers would say that photography is all about understanding light. Photographers create mood and bring out emotion by manipulating the subtle layering of light and shadow. There are two sources of light underwater photographers have at their disposal—ambient sunlight and electronic artificial light, usually in the form of a strobe (flash). What follows is a discussion of what you need to think about to balance these two sources of light.
There are two straightforward ways to light your subject—naturally with sunlight and with artificial, normally strobe light. Without a strobe you rely exclusively on a single source of light, the sun. It takes a great deal of creativity to execute interplay if light and shadow, but a single light source simplifies the thought process.
Underwater photography with strobe lighting is also relatively simple. This is normally accomplished with low ambient light levels, such that the strobe light overwhelms ambient light—overcast days, night or cave diving, or with macro photography when the camera and strobes are very close to the subject and the image is acquired at a small aperture (f/16-32). You can get creative and use two or even three strobes, but you basically set up lighting by positioning strobes on arms, and light is controlled by adjusting strobe position and power levels. The most difficult lighting situation is when you have both natural and strobe illumination and you must balance the two to achieve optimal lighting.
Example One: Sun not in the image. This example is the more straightforward of the two since the background light is normally very uniform, and it is much easier to position the subject relative to the background. First, take a meter reading of the water behind your subject. If your meter is accurate, you will want to overexpose the background by one half to one full stop. This is because the light meter will want to record the background as 18 percent gray, which many will find to be a bit underexposed and drab. I suggest a manual camera mode, and set your shutter speed around 1/60 sec. and shoot several shots at different apertures. Those with digital cameras can shoot a few test shots to see what background exposure they find the most pleasing.
The next step is to position your subject within the background and adjust your strobes to light it the way you see the final image. If you have one strobe, set it above and behind one side of your camera. If you have two strobes, set one on each side of your camera and about the same position relative to your subject. If your subject mostly fills the frame or at least fills the majority of the center of the frame, then TTL automatic exposure might work well for you. If your subject is small or off center, you will need to set your strobes on manual and control the exposure by the power setting of your strobes and/or the distance between your strobe and subject. Begin with your strobes; set the same distance and the same power level to evenly light your subject and cause it to “pop” out of the image. Then try changing the relative ratio of light between each strobe to create subtle shadows. Try bracketing to see what light level works best for you. If the ambient light level is low, then you will need to use a larger f/stop (and have small depth of field) and there is some danger that your strobes will overpower the background. This is when a strobe with manual power settings of 1/8 or even 1/16 power is worth its weight in gold.
Example Two: sun behind the subject. This situation is more difficult than the previous not because of the depth of field, but because of the difficulty in lining up the subject with the “right” part of the background, and the difficulty is deciding in the proper exposure for the background. You actually want the sun to be a bit over exposed; otherwise the image looks dark and moody. Set your camera on manual and set your exposure based on a spot meter reading just off the sun.
With a bright background you will probably want to be shooting manually at f/16 or 22. Your depth of field will be large and your strobes can be set at full or half power depending on the strobe. If your subject is large relative to the background than you may try an automatic setting with TTL metering, but I suspect manual settings will work best.
Establishing the correct composition may be the most difficult part of the adventure. Do you want the sunburst directly behind your subject, or off to one side? In some situations both you and your subject may be capable of moving relative to the sun. This movement not only effects composition, but also the brightness of the background directly behind the subject. You may need to shoot many images and bracket to get the shot your mind “sees.”