Subject:	Classic backscatter (+)
Author:	drdread
Date:	6/24/2002 8:48:27 AM
Message:

You've got psoriasis of the sea, also known as backscatter! This case is not the usual backscatter from having the strobe too close to the lens; this case is from having the strobe too far forward. Your WA lens sees a much greater included angle, and can see the hot spot from the strobe before the beam spreads out as it moves away from the stobe head.

The thing people forget is that backscatter happens all the time, no matter what. All the advice & tricks on how to avoid backscatter are basically designed to move the backscatter out of the lens's sight.

The area near the strobe head delivers lots of scattered light simply because it's the brightest zone. This, BTW, is how slave sensors work; they detect the scattered flash near the strobe head. I suspect that most slave sensors will not work well in air unless they have a clean line of sight to some part of the strobe head's active element--but underwater, you can point them in the general vicinity and they'll work fine.

Anyway, back to your problem: when working with WA lenses, you want the strobe head to be as far behind the lens as possible so the beam can spread out enough to alleviate the scatter. You can also hand-hold the strobe waaaaay out the side, which has the same effect (this is what I do most of the time). Mounting a diffuser might help, although the loss of intensity might be a problem. All things considered, start by moving the strobe backwards...

Good luck,
D

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