Please help me decide on diopter(s) for my new underwater setup, a Nikon D40 in an Ikelite #6804.1 housing with the 18-35mm kit lens and 6" #5503.50 dome port. I've used the setup while snorkeling in some fresh water springs and while it works well for larger subjects, I want to be able to focus closer on small subjects. I have read that a +4 diopter can be used to improve close focus without affecting the ability to focus on subjects farther away.
My two questions are:
1. Is the +4 diopter the correct choice?
2. Is it worth getting a single, much more expensive 2-element, multi-coated +4 diopter (~$100) compared to the much less expensive diopter sets (~$35 for a +1, +2 and +4).
A dome port causes an apparent image that is much closer than the subject. In the Ikelite port manual they show an apparent image at around 12" for a subject that is 3' away. The diopter helps to ensure that the lens can focus as close as it needs to.
I have found through testing that a +4 diopter lens essentially preserves the magnification possible with the lens when using it behind a dome port. For example, if the lens is capable of 1:3 magnification on land, then behind a 6" dome port, you need a +4 diopter lens to get 1:3 magnification.
