I have the 90mm and port, which I understand can also fit the Canon 60mm and metabones.

It's my intent to adapt the Canon 8-15 at some point in the future so the adapter is coming one way or another.

Other than a bit of vignette on longer shots and the ability to shoot two FL on one dive, is there any other reason to prefer the MFO?

I've used the Canon 60mm but not the MFO-3 yet. The 60mm on full frame behind a flat port doesn't give the best image quality. It's useable, but from what I've seen, the MFO-3 should be better. The 90mm + MFO-3 is also wider than the 60mm, and it can focus closer. And native autofocus may offer some advantages versus the adapted 60mm. Adding in the versatility of being able to switch between the 90mm and MFO-3 underwater, to me, the MFO-3 seems like the more desirable option.

The Canon 60mm macro is an APS-C lens and using it on an APS-C system should be decent and gives you 90mm focal length which is +/- a standard macro lens for an APS-C system, while the 90 mm is 135mm equivalent and in my mind a bit long for an APS-C system unless you are shooting mostly very small things. I don't recall but I'm guessing you shoot APS-C Sony?

Assuming APS-C Sony, to make it more difficult to work out what to do, I expect you could use the MFO3 on the 60mm lens as well, I know it certainly works well with my 60mm olympus macro lens and gives you a decently wide view which would be better if you are shooting weedy sea dragon (400mm long ) size critters if you expect to encounter critters in this size range. You would of course need to check it will work - Nauticam have remove dthe EF-s 60mm from their port chart, but the Nikon 60mm macro is listed on the that chart as working with the MFO3 so I expect it probably should work as long as the flat port fits well. You could check for vignetting on land. It might also work with the Zeiss 50mm macro which has a few fans on this site as I recall, but again you'd need to check for vignetting.

Having said all that the big attraction for me is carrying a wider view with you when shooting macro.

The Canon apparently only vignettes slightly on Sony FF bodies and only at distance. The idea would be for a wider lens.

Interestingly the little 30mm Sony ASP-C macro covers a larger area of the FF sensor than you'd think and is quite fast. So if I can find a port to fit it that would be a wider option that can basically focus on the glass.

Edited 26 minutes ago26 min by Grantmac