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.
Edited by Marsh, 20 November 2017 - 10:38 PM.