Shims needed?
The Q&A copy blurb from MegaVision below says it best. If you google "Digital Back Shims", you will see it is often needed for the so called "Universal" digital backs.
As ex&le, I could never get my Kodak 645C back to focus the 80/2 @ f/2 on my Contax 645 ... I bought magnifiers, got the Contax WLF, nothing worked ... so I sent the entire rig (camera, 80/2, and digital back) to Kodak Digital and they shimed the back to perfection.
Here's the blurb:
Q: Although I am sure I focused properly with my camera, the image seems to be out of focus...
A: Your single-shot digital back may not be shimmed correctly. The digital back contains shims between the actual back and the adapter plate. Although MegaVision uses the specifications supplied to us by the various camera manufacturers (Hasselblad, Maymia, etc) AND tests each and every digital back before it leaves the factory, there still is a chance that your individual camera may be slightly different and require a custom fitting. This can be caused by simple use; your camera may "loosen up" a little after years of use. MegaVision offers to our customers a custom fitting service. If you send your single-shot digital back along with your camera (please include lens & view finder) to the factory we will custom shim your camera to correct specifications.
One of the reasons I like the so called "closed system" offered by Hasselblad is that the Back and camera are matched at the factory. I suspect ( hope ) this will be the case if you order a Hy6 camera from a digital back maker like Sinar or Leaf.