Originally posted by ElJamoquio Equivalent DOF: a little less than one stop (multiply the F-stop number from 36x24 by 1.3).
That's only true for moving from one film format to another, not film to digital.
And that's my entire point. I did the "diagonal conversion" of 1.6 (medium format film of 41.5x56mm to 24x36mm) thinking that would work, but it didn't (as my original post explains). I went from FILM medium format to 35mm DIGITAL. Just factoring in the diagonal (crop factor, you might say) is not the only factor. Pixel pitch matters. How much exactly, I say 3 x pixel pitch, which works for very small apertures. It may be 2 x pixel pitch for the "sharper" apertures like f/5.6 and f/8.
Please do physical tests and find out for yourself. You'll be as surprised as I was. You can test it without a 645D, though.