I see "full frame" mentioned a lot in this discussion. Full frame, or a 35mm film size sensor, in digital 35mm cameras is something hampered by physics. It's hard enough to do in an SLR (Canon uses a software cheat to get acceptable corner results, Nikon [as I understand it] didn't go the cheat route since it deemed enhancing the underexposure at the corners of the frame wasn't acceptable) and even harder to do in a rangefinder because of the shorter lens rear element to film plane distance.
The physics comes in because light passes through the lens and, as you get to the edges, hits the sensor/film at an angle. Film doesn't care that much about the angle at which the light hits. Sensors like the light to hit head on. From what I've read, the light fall off at the corners in the Canon 5D is something like 2 - 2 1/2 stops. Software in the camera is used to compensate for this vignetting.
Many rangefinder lenses extend into the camera body, making the angle when the light hits the film/sensor even steeper.
While microlenses could be used to alleviate this problem, this would probably eliminate some lenses from use at all and be a compromise at best for others.
A smaller sensor (APS size) allows for true RAW files (no in camera processing). However, it does force a redesign of wide lenses due to the so-called crop factor.
Peter