I'd think you would want a larger rangefinder in order to be more accurate?
Perhaps the simple, compact, light, cheap thing to do is to not have a built -in rangefinder and use a shoe-mounted separate one instead (like an early Leica did). It would allow you to have different scales mounted to the rail for each lens, and the moment spent transferring the rangefinder distance to the focusing against the rail mark would be minimal anyway.
Might be time for a new thread rather than continuing here....
Bookmarks