What it is: When particles suspended in the water are illuminated by a strobe, they can appear as distracting white particles in your photo. The particles can be either small specks or larger white blobs that take on the shape of the lens aperture, depending on how focused the particles are.
What causes it: If the strobe is too close to the lens, particles will be illuminated and exaggerated by the full frontal light. Sometimes the water is simply too dirty to get a photo free of backscatter no matter where the strobe is placed.
How to fix it: The angle at which the strobe is held can make a significant difference in minimizing backscatter. It is far better to move the strobe away from the lens, typically at a 45-degree angle above and to the side of the subject. Remember the effects of refraction under water, and aim your strobe slightly beyond your visual perception of the subject’s location.
When the water is very dirty, particles may still appear in your photo regardless of strobe placement. Note in Photo 1 that the particles are most apparent in the upper portion of the photo, leading me to assume that a good wide-angle shot that includes open water was probably not possible under these conditions. Instead, I would have been better off using different optics to shoot close-ups of subjects like the starfish. By doing so, I also could have reduced the amount of water between lens and subject. I also could have found a background better suited to camouflaging the particles.
Overall lessons on backscatter: (1) Aim your strobe properly to minimize the number of particles you illuminate in front of your lens. (2) Let local conditions help determine the best kind of photos to take in order to minimize backscatter.
When a strobe is too close to the lens and provides full frontal light (left), particles visible to the lens are illuminated, causing backscattter. To minimize backscatter (right), place the strobe at a 45-degree angle above and to the side of the subject, thereby illuminating the sides of particles not visible to the lens.
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