cdiver2
Contributor
The OP seems to be with regard to a strobe. This answer/link seems to be with regard to dual strobes. I shoot a lot with an INON WAL, using a single strobe with 100 deg coverage. To cover the entire image, even with a diffuser, the strobe needs to be pointed at the center of the image.
As the subject distance changes, the aiming of the strobe needs to change. One theory is that light falls underwater, so as distance to subject increases, aiming a little above center looks like the strobe burst is centered.
Backscatter is greater as the angle of strobe is closer to parallel with the sensor view. Using the internal flash will result in the most BS, since it is closest to parallel. Holding your strobe as close to the lens as possible will give the most BS with a strobe. The farther away from the camera you hold the strobe, the less BS you will see. Using the edge of a single strobe on the subject will result in one side of the image being much darker than the other. The amount of BS between the subject and lens will be the same, but like most of the entire image it will not be as bright.
Using a single strobe is the most common form of underwater strobe photography. Dual strobe systems are for a small minority of divers, for various reasons. Many of the most famous underwater images in the world were taken with single strobes. Quite a few of the most famous underwater images in the world were taken with internal flash. Some of the most famous underwater images in the world were taken with no strobe/flash (ambient).
In general, I try to aim the strobe at the center of the image. If backscatter is an issue I put as much distance between the strobe and lens as possible. If there is a lot of particulate matter in the water, some backscatter in the image gives the image a more realistic feel. In many eyes, an image with absolutely no BS has a sanitized feel and lacks depth. The same is often said about the lack of shadows with dual strobes. Art can be accomplished with a pinhole box, just ask Ansel Adams fans.
daveread.com - Underwater Photography: Aiming Your Strobes
