> > I don't quite buy that. What determines the size of the viewfinder image is the > size of the frame the screen sits in (as long as we are talking +/- a > millimeter). Make that frame a little bit larger and you have a 100% > viewfinder. Of course, all elements that attach to the mirror box have to be > 'accurate' but I don't see why that would be so difficult here.
Ah, but that's precisely what *is* difficult. It's not just the size of the viewfinder - it's position the boundaries accurately. It's easy(-ish) to make a 95% viewfinder, because you only have to position the viewfinder region to +/- one mm. Make the frame a little bit larger, though, and you don't have a 100% viewfinder; if you're off by that same 1mm you might have a viewfinder that showed 97.5% of the image, cropping off 2.5% on the left, and showing an extra 2.5% on the right that wasn't part of the true image area. This would be bad. If you can see it through the viewfinder, people expect it to show up on the image. The extra 5% allows for a certain amount of inaccuracy. To get a true 100% viewfinder (no more, no less) would require at least an order of magnitude more accuracy - everything would have to be aligned to a precision of 0.1mm or better. That would require a camera considerably more rigid, and manufactured to much closer tolerances, than consumer-level prices can support.
