Just a comment about the ratios. 6x4.5 and 6x7 are not the precise dimensions of the negatives. Trusting my memory (sounds dangerous ):Originally Posted bygame

Mamiya 645: negative size 56 x 41.5, which gives a ratio = 1.35

Mamiya RB67: negative size 56 x 69.5, which gives a ratio = 1.24

Don't know about Pentax 67, but it's probably a bit less square than suggested by the term "6x7".

Knut