You've finally bought a strobe for your underwater camera. Now your photos show the true colors of the reef. But where did all those little specks and white spots come from? What you are experiencing is a phenomenon known as backscatter, a condition caused when the light from your strobe bounces off small objects suspended in the water. The culprits could be sand, coral, dirt, air bubbles, decomposing matter or even tiny animals. Regardless, the results are the same: spotty photos. Backscatter is completely unique to underwater photography and it's one of the most difficult problems to control — you can even encounter it in the clearest of waters. Here are a few tips that will help.
Move the Strobe Away from the Camera
By moving the strobe away from the camera lens, you light fewer particles directly in front of the lens. Moving the strobe also reduces the ratio of light hitting the particles, as compared to the light falling on the background.
Balance Flash and Sunlight
When in really dirty water with lots of particles, set your strobe so that it matches the sunlight. This usually requires that you use a wider lens opening and reduce the flash output so it will match the natural sunlight in the scene. If you use TTL metering, you will have to open the lens until the correct exposure is near flash sync. If you use a manual flash, take a meter reading of the sunlight, and then use that f/stop setting for your strobe.
Use a Twin Strobe System
Twin strobe systems can reduce the effect of backscatter by evenly lighting the entire scene, including the background. Both strobes should be positioned out at 45-degree angles, so that the flash coverage is even and no hot spots are created. You can also point one strobe directly at the subject at medium or low flash output, and a second strobe toward the background at high output. The higher output on the second strobe is needed because more light is required to reach the background.
Use a Wide-Angle Flash
Using a wide-angle flash reduces the concentration of light striking the backscatter particles and better illuminates the background. If the particles in the water become dense, just move the strobe farther from the camera. When you move the strobe, make sure that your re-adjust its direction to keep aimed in the same direction as the lens.
Use a Wider-Angle Lens
When you use a wide-angle lens, it allows you to move in closer. Less distance means fewer particles between the camera lens and your subject, and less backscatter. This technique works with both wide-angle and macro photography, assuming your subjects will allow you to get close.
Backlight the Subject
If you find the backscatter so concentrated that all other solutions do not work, you can place a strobe behind your subject. This only works if the light will pass through the subject, or wrap around it enough to give it shape, color and dimension. We often use this technique with large cold-water sea anemones when the visibility drops to about two feet.
Turn off the Strobe
Sunlight provides even lighting that results in minimal contrast between particles and subject. If backscatter is very heavy, you may want to turn off your strobe and expose your film using only available light.
So, what happens if you try everything and you still end up with backscatter? No matter how far your strobes are from your lens or how clear the water is, often, a few annoying specks will show up in your images. That’s where post processing is invaluable. The clone stamp and healing brush in Adobe Photoshop does a great job of removing backscatter. However, if you have a lot of backscatter, the painstaking process of sitting and removing hundreds of particles in post processing is usually not worth it, so it’s important to try and minimize backscatter as much as possible when shooting.