To try to answer the original question, when set to a
specific focal length, the angle of coverage of the
flash should roughly match the diagonal angle of
coverage of the lens. But at wide angles I think
there's some fall off in the corners, so it's probably
slightly less.
BUT! That is in air. Water changes things because it
refracts light differently than air. You've got
refraction occuring at the light - to - glass boundary
and the glass - to - water boundary. I'm not sure how
that's going to affect things. On top of that, even
with a reflector, objects on the glass side of the
aquarium are going to get more light than object
farther away.
My suggestion would be to make a few more trials. Put
several FIXED items in the aquarium for the test. On
on the reflector side, on in the middle, and one near
the glass side. At a fixed focal length of the lens,
manually adjust the coverage of the flash, from a bit
narrower than the focal length of the lens to several
steps wider. You should be able to get a pretty good
idea of coverage that way.
HTH,
=====
Bob Meyer
I wish I knew what I know now, when I was younger...
http://www.meyerweb.net/epson
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