Well, I'm almost there with the housing purchase..donations gratefully accepted!
I'm getting confused about diopter use. I understand that they are necessary for zoom lenses because of the short minimum focussing distance, but are they necessary for single length wide angle lenses? I have a Nikon 20mm, which focuses to 10", and Sigma 24mm macro, which focusses down to 7". I'll be using an 8" dome port on an Aquatica Housing. Any clarification would be appreciated! There are some benefits to my Nikonos, after all...it doesn't confuse me, at least.
Marli

You won't be needing a dioptre on the 20mm or 24mm with the 8" dome.

Thanks, that's what I thought, but had heard conflicting info.
Cheers,
Marli

Following URL describes reasons: http://www.ikelite.com/web_pages/qdiopter.html

Thanks for the clarification, Ike.

I'm using a 17-40L zoom, which focuses to 28cm, so I figure it should be fine focusing on the apparent image using a dome port without a diopter.

Maybe your site should say:

"Cheap, crappy, consumer-grade multiple focal length ("zoom") lenses usually will not focus close enough to provide full range operation behind any dome without adding a +4 diopter close-up lens"