Subject:	Screwball optics
Author:	drdread
Date:	4/14/2002 11:52:15 AM
Message:

The distortion you're seeing is a result of a mismatch between the screw-on lens and the built-in lens on your camera. There's probably no fix for it. What's worse, the housing will probably exacerbate the problem, although I can dream up some situations where the reverse might happen. It's a stretch, though. :)

Minimizing the problem: you might try higher aperture numbers and see if that helps. If it's simply a minor focus issue, extra depth of field will probably help. Unfortunately, the dome port on your housing (it does have a dome port, right?) will likely introduce an extra source of mismatch. Worse still, the dome will create a virtual image that's ~10-16 inches from the lens, which means the lens has to be able to focus down to that limit or the whole idea is a no-go. :(

FWIW, my housed system exhibits similar behavior with the 20mm lens and the dome port. The corners of the frame go soft, as you can see in the pictures I've linked below. The funny thing is that it doesn't happen with the 24mm lens; I suspect that the dome port matches the position of the 24mm lens nearly perfectly, while it's a little off for the 20mm lens.

The most common scenario is for the focus to work better for far objects than near objects. This is a design parameter in most cases; the thought is that people buying WA lenses want to photograph mountain ranges, prairies, forests, etc., and that you'll do those things essentially focussed at infinity. That is, the designers consider topside uses, and topside users typically don't do CFWA work. Sure, some people do, but 99% of topside WA is scenic/panoramic in nature. Hence my recommendation that you try some WA shots focussed at infinity, just as an experiment. Clearly that's not going to help you U/W, but digital "film" doesn't cost you anything. :)

The sensor in your camera is probably half an inch across, which really makes the WA have to work hard to cram the whole WA scene onto the tiny little sensor. This is one of the reasons I have held off going digital; WA is 80% of my interest in photography, and digital setups basically suck at this. Larger sensors ("coming soon") will mitigate this problem.

By the way, I love the composition on the top photo. Super nice job there.

D

screwed up focus:
Look around the corners
Upper left corner, lower right corner

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