Go away for a few days, and look at all the talk...
Anyway, fpoole, your post #9 is a pretty good summary of what works.
Except we didn't mention the curved-field issue...
In the illustrations following, I've made them to scale, using numbers for a 14mm f2.8 Nikkor. Depth of Field is to scale as well.
Dome ports generate a virtual image that is curved. So, a 8" dome generates a virtual image that is about 16" from the focal plane, in a curve that matches the dome:
A typical wide-angle lens is rectilinear, and focuses in a flat plane. Here's the 14mm lens, with the depth of field that is accecptabally sharp:
With a small dome, note that the edges (usually the corners) of the virtual image fall outside the Depth of Field, and go soft:
However, in this case, an 8" dome gives the depth of field a bit more room to cover the entire frame.
Note: the focus is actually short of the virtual image!
So you can see, there are a few more variables than just that the lens can focus at the virtual image. You actually have to have appreciable ability to focus shorter than the virtual image.
At the risk of being long-winded, I should point out that fisheye lenses have focus planes that are curved (Not flat, like the 14mm illustration above). This allows very sharp images when behind a dome port, without a diopter. Of course, the image has fisheye distortion, which is a matter of taste as to whether you love it or hate it.
Reputable manufacturers, like Ikelite, have actually tested their lens/dome combinations and are a safe bet.
Hope this didn't make things muddier!
All the best, James
