Quote Originally Posted by Gerald C Koch View Post
The accuracy of the viewfinder of an SLR is determined by the size of the mirror. There are several competing problems here. If the mirror is designed to provide 100% coverage the inertia of the mirror becomes a problem. Camera shake also increases with mirror size. Most SLR's are therefore a compromize and provide less than 100% coverage.
I don't think it's about mirror size only, or even mainly. The increase in area, thus mass, of a mirror which reflects 95% of the frame area, to one which reflects 100% is slightly more than 5%. I think it's much more about precise alignment. If a viewfinder shows less than 100%, the viewfinder image can "float" within the actual frame, which is borne out by tests done on cameras with less than full coverage which often show the viewfinder image to not be centered. A 100% viewfinder must be very accurate to be useful, and that is harder to do and more expensive.
Some quiet, low vibration cameras have nearly 100% viewfinder images. The OM-1 was one of the first to address mirror noise and vibration in a determined way, and its coverage is something like 97%.