I'm working on a project and have come into a bit of an issue. I am working on a custom back for my Hasselblad V but cannot seem to get the imaging plane close enough to the proper film plane registration distance by a couple of mm. My thought was that a +diopter accessory lens (a close-up lens) "pulls" the nodal point forwards, away from the imaging plane and towards the lens, which is what permits a lens to focus more closely with the standard focusing helicoid; +diopter lenses are typically plano-convex or double-convex. Would a -diopter accessory lens (a plano-concave or double-concave lens) "push" the nodal point in the other direction and allow focusing past infinity with the standard focusing helicoid?

If so, where could I get such lenses in the thin, large diameter sizes necessary to fit a filter mount?

Obviously, the best thing to do is to get this custom back's registration down to the right dimension, but if it turns out that isn't possible I need to shift the infinity focus imaging plane somehow.

Thanks for any ideas!

"Laws of physics be damned, just do what I want, beastly things!"