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.