I ran into a great deal of trouble researching and building this set up so I wanted to share in case it helps save someone else a few weeks of reading countless articles, books, websites, and company calls.
I already owned a Sony so I went with what I felt the best lens is that still fit based on what I measured in the extensions. Again: This is not a supported lens from Ikelite at this time but it works and you can add in a zoom ring if desired with a simple mod. My guess is they allow for some fudge room the on stated width allowed for port extensions (~1 mm) to avoid lenses not working in cases where measurements are not exact or help insure the standard included zoom ring will fit. Facts for this lens setup in case you care to try a different one (sample photos below). - The entrance pupil or nodal point for this lens measures at 62-68 mm (10-20 mm zoom) from the base (ends up near the gold ring). - 8" Dome. - Port Theory says that as a general rule the entrance pupil should rest at the same distance from the dome as it's radius or 4" in my case. Port theory and reality of what looks good aren't always the same but is a great starting point. - I ended up using the 5510.16 extension at 51.76 mm length along with the 5510.50 extension ring at 32.66 mm. This is short of the optimum length but to go any longer means you'll see the port extension in the frame at the corners (see photo). What needs to be done to make the zoom ring work: Cut off the springs which hold the grabby part to the zoom ring because it's too fat with them. You can then hold the two remaining pieces to the lens with double sided tape if you like but not needed. I'm told there may be a low profile zoom ring component available as well. Alternatively, you can just use the right thickness of adhesive foam on the inside of the clear zoom ring (same as what comes with it). Often there is an optimum zoom for clarity in the corners on a rectilinear lens... so if you find it just set it and forget the zoom ring all together. Other extension dimensions per Brett w/Ikelite (thank you). This is the total physical length of each. Extensions: 5510.16 = 51.76 mm 5510.22 = 67.64 mm 5510.24 = 71.13 mm 5510.28 = 93.31 mm Extension Rings: 5510.50 = 32.66 mm 5510.54 = 44.95 mm 5510.58 = 58.14 mm Sample Photo Showing cropped photos (to left side) of my Test results. Keep in mind the optimum zoom for the last photo would have been somewhere between 10-20 mm and not right at the 10 mm I shot but I wanted consistency. From left to right goes what other photographers suggested, middle was the manufactures best guess and right was what I calculated.
My calculations to find the needed extension ring: For purposes of my calculations I always went off the inner radius of the dome vs. the outer, I have no idea which one is best. The entrance pupil or nodal point for my lens measured at 62-68 mm from the base. The dome is not a full half sphere so my best measurement puts the physical inner distance at ~67 mm from glass to the outer flat base of the dome port. This means it's still ~34.6 mm short of the true 4" (101.6 mm) inner radius of a complete half sphere. To find length of extension needed from camera body to dome I added the nodal point to the short fall of dome radius. EX: 34.6 mm (+) 62 mm (=) 96.6 mm of needed extension length but doesn't account for the extension overlap with the dome due to o-rings and threading. Next, account for the fact that the port extension doesn't connect flush with the camera body like the lens. My best guess is the entire physical port extension connects at a distance ~20 mm from the camera body. EX: I take 96.6 mm (-) 20 mm = 76.6 mm. Then account for the overlap mentioned which was ~13 mm so 13 mm (+) 76.6 mm (=) 89.6 mm of total physical extension length needed at 10 mm zoom or 95.6 mm at 20 mm zoom. Again, I opted not to go with the extension 5510.28 even though it's likely the right length because it was visible in frame at 10 mm zoom. It did provided more clarity then the other extensions tested but it wasn't worth it to me at the time. In hind site is was a rookie mistake to worry about the zoom function instead of just setting my lens to 12mm before diving. You can see what I mean in the sample photo.
So what do you think, did I get it mostly right or would you go back to the longer extension and slightly zoom the lens?