Ok thanks, as you seem to have found out the nodal points of the domes must be corrected with extensions to coincide with the lens' nodal point otherwise the results will always be bad.
So there could be hope for the 4.33" maybe? Anyone that have a couple of 20mm extensions lying around to test?
Edit: could i ask you to measure the 4" port length from base to, say inside top of dome?
I quickly tried the 3.5" and 4.33" domes with a 30mm extension on the EM-5 and the 9-18mm and a good +4 topside diopter.
I did not upload the pictures but the findings:
- the 3.5" dome just sits too close (maybe a mm) from the lens and diopter. The result is just unacceptable unless you close to F11 or so.
- the 4.33" dome is acceptable once you correct for distortions (both chromatic and focal). Good results at f8.
However the 4" semidome still behaves better and for some reason it can focus much closer (like 20% or 25% better magnification). Good even wide open!
Maybe with 2 20mm extensions make the 4.33 worthwhile but at this point I do not see a reason to use the 3.5" or 4.33" domes with the 9-18mm.
The trials have shown that at least a +2 diopter will make the 9-18mm lens work well in the 4" semidome. I wished I had known before as I have used this lens underwater very seldom....!