Even the clearest water has at least some particles suspended in it — sand, silt, plankton, who knows what — and most of the time, in most places, the water isn’t really all that clear. In fact, during a plankton bloom or after a storm that has caused a lot of runoff, you might just as well leave your camera on the boat. Light from a camera strobe has a nasty habit of bouncing back from any particulate matter that it encounters between the camera lens and the subject. The result is the bane of underwater photography: backscatter. Your images will look as though they were taken in a blizzard.
The backscatter problem can be mitigated in several ways. Being as close as possible to the subject is one strategy. Nearness minimizes the volume of water between the lens and the subject, so the number of suspended particles is proportionately reduced as well.
Another strategy is to take advantage of the Law of Reflection by aiming your strobe(s) at an angle such that the light rays emitted will bounce (reflect) off any particles in some direction other than back toward your lens. Usually this means lighting the subject with one or more strobes positioned off to the side(s) of the subject, as opposed to aiming them straight ahead.
Remember, though, that the Law of Reflection assumes that the incident ray will be reflected from a flat, mirror-like surface. I think it’s safe to say that, in the underwater world, the particles from which the light rays will be reflected are likely neither mirror-like nor flat. Instead they can be virtually any shape, and that irregularity will affect the angle of reflection. So, this strategy won’t completely eliminate the potential for backscatter, but it can go a long way toward minimizing the problem.
Of course, these days you can use photo editing software to correct for backscatter, at least up to a point. A few ‘dots’ here and there usually can be made to disappear, but if you have an image that really does look like it was shot during a blizzard, eliminating all the backscatter can be a monumental — and sometimes impossible — task.
Below are four macro images that originally had at least some backscatter. Each has been corrected using the Microsoft Digital Image Editor scratch removal tool.
The first image had only a few dots of backscatter that were very easily removed. The subject is an arm of a crinoid — probably Heterometra savignyi — from the Red Sea.
The second image, below, is a White Speckled Hermit Crab (Paguristes punticeps), a Caribbean species. It was photographed in the Cayman Islands.
Next is a Bryozoan species from Hawaii, Crisina radians, also known by its common name, Tuning Fork Bryozoan. These fan-like colonies are very small; the one pictured was approximately 1.5 cm (about a half inch) wide.
The final image shows one of the most common nudibranchs found in Hawaii, Phyllidiella pustulosa. By the way, in natural light those pink nodules look green. The pink color is apparent only when the creature is lighted artificially underwater. This individual was about 5 cm (2 inches) long.