The Zeiss 12mm lens is absolutely fantastic underwater. It's known to be sharper than FF lens from both Canon and Nikon. Being a rectilinear design, corner sharpness is always a topic of conversation with most photographers suggesting a larger dome, plus some of the suggestions listed in this thread. I have a modified Zeiss 12 where I removed the bell shaped shade supporting structure and replaced it with 67mm filter rings (the lens doesn't need the shade structure to function) . This has allowed me to use the lens in both a Nauticam 4.33 and 7" single piece domes. For travel purposes, I wanted the 4.33 to work since it's tiny and well suited for CFWA photography. I bought the much larger 7"dome (yet small by some standards) that was designed for the Sony 10-18 WA to test and make comparisons. . Given the 7" is not a fisheye dome (like the 4.33), I expected it to have much sharper corners. In testing, it did have sharper corners, but not by as much as I expected. In fact, to my surprise, the 4.33 dome is almost as sharp and much, much better for CFWA. I can actually focus on objects very close, within an inch or two from the face of the dome! Obviously, not so with the much larger 7" dome.
I ended up using the 4.33 for travel purposes and for CFWA. To get even more use out of the dome, I actually configured a 67mm Nauticam swing mount (from my old port 72) to fit across the face of the dome. I use this configuration for my Sony 18-55mm and can swing a CMC-1 15+ close-up lens at the 55mm end for some great super-macro or 18mm WA on a single dive. To get the correct dome placement for the Z12 that Alex is referring to, I added a 20mm extension to the 4.33. This was also needed for the Sony 18-55 longer length at 55. It seemed to put the Z12 in the ideal spot for decent corner sharpness. I usually shoot f11-16, sometimes more, but too much and you end up with soft shoots from aperture refraction.
I've considered at some point of creating a graduated sharpening filter to match the 4.33 optics in Photoshop that will gradually sharpen the corners very slightly for all shots with the Z12 and and S18-55. The shots below are an untouched jpeg's made from a RAW file on my NEX7 and Z12. The lens is sharp and the corners look pretty good for a small 4.33" FE dome.
Looks like my photos didn't post? I need to study how to upload media to the site to use it? Hope the info helped though.
Thanks for the info on the manual focus for the S90 on the APS-C platform. I recently discussed this subject of equipment with Kevin at Reef Photo and he completely gave me different product descriptions than what you described using the Port 55 with a centering ring. I will research what you have shared to make sure i get the latest and best working system for the S90 on APS-C. As we and others have found out, the S90 works fine with +5 diopters, but has difficulty with +10. Unlike you, I really didn't see much improvement with the SMC-1, but I did with a CMC-1+15 !
I recently I purchased one to experimented on my modified 4.33 port where I can shoot WA and M on the same dive (67mm swing grafted on to the ports shade) for both my Sony 18-55 (w/20mm extension) and my Sony/Ziess 16-70 (w/17 & 30mm extensions combined) using a custom focus gear I made (thank you for heading me in that design direction with tubing and timing belt :-) The CMC-1 worked best with the S18-55-really sharp and fast focus. The Z16-70 optics just don't work with large diopters so I gave up on trying to use it as a single dive lens for both WA and M. Too bad, as the added wider FOV and increased magnification from the 70mm would have been nice, but performance just wasn't there on the macro side and it didn't focus as closely as the S18-55. I've finally abandoned using the Z16-70 for this reason. I will be putting the custom designed gear and Z16-70 up for sale soon for someone who wants to use the Z1-70 for video, as it works great with the custom zoom gear in the 4.33 compact port.
Okay, back to the S90 Macro. The surprise I came across in working with the CMC-1, was in reversing it with a 67mm M/M ring on the Sony 90 macro. The S90 focused sharply for the first time and would lock on fairly easily especially when I set the limiter for short travel. While reversing it lowered the magnification by a hair, It increased the working distance which was an added bonus. These were the same results for my Canon 60 macro. Despite reports the CMC-1 doesn't focus optimally with the C60, it works beautifully reversed, but focuses much slower with the Metabones. I've decided I want to use the native Sony 90 macro (effective 135mm on APS-C) for super-macro now since I now have a +15 diopter that works successfully with it. Auto-focus is handy in current so I hope to get decent response with the S90 and CMC-1on my NEX7, as it works good on the surface. I'll supplement focusing with Phil's method above, which is what I use on most occasions.I will soon be selling the Canon set-up.
My final APS-C system will now be the modified Z12 and S18-55 in the Port 4.33, with the S90 for SM. Should I keep the Z16-70, it will work fine for video in the 4.33 port. A nice compact system.