Last June, I purchased an Isotta housing for my D500. I got their standard 60 macro port and a 6.5" dome port plus enough adapters to manage 60, 105, 10.5, 8-15, and 16-85 lenses. My first trial with the housing was at BHB for a week in June. One of the things I noticed right away was that I could not see critical focus through the viewfinder. With macro, I typically get focus close, then move in/out to achieve critical focus on the plane of choice. Couldn't do it. I relied on single point focus and putting the spot where I wanted it. Got results, but really did not like not being able to "see." I tried several things, but nothing improved.
I discussed back and forth with Backscatter over the following months - even sent the housing back so they could see the problem. Of course, everything looked fine to them. They lent an Inon 45º finder for a Sea of Cortez trip in September, which worked great - crystal clear focus. During the trip, I had several other photographers try the rig with Isotta's viewfinder. Several could focus just fine, others could not. The difference - bifocals. Those with long-arm disease could not focus; those with younger eyes could.
Has anyone else seen this problem with SLR housing viewfinders? My thought at this point is that the Isotta housing creates an apparent image within my "need bifocal" range. I tried focusing through bifocals and have to say that didn't work out so well either. The only dSLR housings that I've used to date have been Ikelite and never had this issue (I still have my D610/Ike housing).
Backscatter is throwing in the towel. They've not heard feedback from anyone else with a problem, so it must just be me... Right now, their solution is for me to buy the Inon finder. It was nice, don't get me wrong, just don't think I should be forced into that purchase.