Backscatter, Cause and Cure

	Strobes placed wide, aimed straight ahead, leaving a maximum amount of dark water between lens and subject, thus minimizing backscatter.
	Strobes placed close to camera, aimed out, leaving some dark water between lens and subject, producing a small amount of backscatter. This technique works fairly well, but not as well as the wider placement above. If you have or prefer shorter strobe arms, this technique will minimize backscatter. As you can see from the photo at left, there is a bit more backscatter than the wide placement described above, but far less than the other placements described below.
	Strobes placed wide, aimed in at subject, illuminating the water in front of the lens and causing a significant amount of backscatter. This is the way most of us instinctively aim our strobes: directly at the subject. It illuminates too many particles too close to the lens.
	Strobes placed close to camera, aimed straight ahead, producing a fairly large amount of backscatter. It is better to aim them out if you can't move them farther away from the lens. This diagram and photo demonstrate why those little point-and-shoot cameras produce so much backscatter.

Backscatter is caused by strobe light reflecting off particles in the water. The closer the particles are to the lens, and the larger the particles, the greater the problem. The solution is to position the strobes to illuminate as little particulate matter as possible, especially that which is close to the lens, while still illuminating the subject. Small particles a couple of feet or more from the camera may create a haze, but won't usually cause the annoying white spots known as backscatter.

The four photographs shown above were taken in the same place one right after the other. The water was pretty clear, but I chose to take the series of photos anyway. The use of good anti-backscatter lighting techniques is much more important when there are more particles suspended in the water. I plan to repeat this experiment in poorer visibility when the occasion presents, but for now, these photos will suffice.

In the uppermost photo, I placed the strobes as I normally would, to the left and to the right of the lens, and each about the same distance either side of the lens as the lens-to-subject distance and facing straight ahead (not angled in). In this case, the strobes were placed almost three feet to each side. (This distance formula only works if the strobe's coverage angle is greater than 90°.) The vis. was about seventy feet, and there was not really a lot of particulate matter in the water.

The next three photographs and accompanying diagrams show other typical strobe placements and their results.

The second-best strobe placement is a bit closer in and angled out. The diagrams show how much water near to the lens is illuminated by each strobe placement. The amount of illuminated water near to the lens is directly proportional to the amount of backscatter in the resulting photos. From these diagrams it is clear to see why those little point-and-shoot cameras with the built-in strobes located right next to the lens create images with so much backscatter. It is also clear to see how using long arms and wide strobe placement to "edge light" the subject while leaving as much dark water between the camera and the subject is the most effective technique.

The following photograph shows a typical wide angle camera system with long strobe arms folded in. The arms can easily be extended out to about three feet to each side of the lens for low backscatter lighting.

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