Am I correct in thinking that there are two 50mm lenses for the Mamiya RF range? One is an f4 made for the Mamiya 6 and the other an f4.5 made for the Mamiya 7/7ii? If so, I have two followup questions: 1. Does the Mamiya 6/f4 50mm work on a Mamiya 7 body? 2. Apart from the small difference in widest aperture, is there any significant different between the two lenses?