I'm thinking about picking up one of these monsters. The price on the used market for these things is very low now, and the quality of the glass very high. Looks like a win/win situation for getting into a 6x7 SLR system. What I'd like to know is, what are the differences between the Pro S and Pro SD models? I am interested more in the Mamiya than the Pentax 67 models. The leaf shutter lenses afford much more flexibility for lighting with strobes.