WG,
Unfortunately, extrapolating doesn't work. Because of differences in sharpness and film flatness between 35mm & MF gear, as well as scanning tolerances, I have found that 22MP backs exceed what is achievable with 6x7 MF gear. And don't get me wrong....I own an RB67. I compared a while ago scans shot on Astia F vs a 22Mp back from Imacon. While it is possible that the film holds more resolution.....it's not usable as at magnifications large enough to see it, you become overwhelmed in grain. The human eye responds quite well to accutance with regards to perceived detail & sharpness. You are able to increase accutance from a digital source through USM to a much greater extent than film because of the fact that digital sources lack noise. ...which would be increased far more using USM from a film source.
That is why you'll find a 48MP betterlight back easily equals 4x5 film. It is also why on a 16x24 print, an 8MP source exceeds 35mm film when many people throw around numbers like 10,12, even 25MP. Give it a try yourself. Print out some 8x10 crops from a 16x20 film & digital source. I've found whenever I do this for people, they are really surprised how well a digital source can do.
Of course, if all you're doing is 16x20, you'll do well with any of 4x5, MF 6x7, or a 12MP digital source.
Best regards,
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