Hi Driftc,
I think you are correct to say the SMC-1 is the better choice if you intend to house the Sony 90mm macro. I don't believe you have mentioned which camera you are using but the higher the MP's the more likely to see flaws in the lens. I have attached some images to give you an idea what you get with the 90 macro and several C/U lenses. The Coca-cola cap is about 30mm across first at 1:1 and then at 1:1 with the SMC-1.
The two Jaw fish are at 1:1, the first is full frame and the second is with the Sony A7R IV set to APS-C. FF is 61MP and APS-C is 26MP.
The Shrimp is at 1:1 with the Aquatica ACU +10 C/U lens. The coral polyp is with the SMC-2 which has little to no DOF and is very hard to light.
While all of the tables and formulas are great information closeup lenses are all but useless if they are not placed very close to the camera lens and even more relevant used to exceed the magnification of the camera lens being used.
It has been my observation both while teaching classes and just looking at images on line that many are adding the closeup lens far before the prime lens has reach its max magnification. Why use a closeup lens with a 1:1 macro lens while the lens is still in the 1:2 range. I have seen many images posted of things like arrow crabs where all of the legs are in the frame and the poster has used a CMC-1 or SMC-1 to take the photo. Arrow crabs are just not that small and you are waisting the value of the expensive macro lens to tate a shot that could have been done at 1:2 or 1:3 with the macro lens. Also no AF macro lens gets all the way to 1:1 using auto focus. If I am going out to shoot with C/U lenses that extend magnification beyond 1:1 (life size) I set the camera to manual focus and the lens to 1:1 before I close the housing. finding the proper subject matter then becomes the issue and w
hen I find a good subject I find the focus range at 1:1 then add the C/U lens and move the camera forward until I finds proper focus. Because DOF is very shallow as has been repeated several times above, finding sharp focus is just a matter of seeing an image go from very soft until it pops into focus. This takes time in the water to master but it is worth the practice.





