> Hmm - I agree with all you are saying, except for the tolerances. The ME > Super mirror box that I am looking at right now is a cast aluminium part. It > attaches to the body, to the mount and to the prism. If it is only machined > to *standard* engineering tolerances of 0.1 mm then it will easily position > the screen frame within +/- 0.2 mm of the film window (or relative to the > sensor) and that is all that is needed to reasonably center any frame size > (+/- 1 mm would be really awfull).
Don't forget the mirror also has to be at *exactly* the correct angle. Is there any foam backing behind the mirror? That's another error to be added in (and probably one with rather more than 0.1mm variation). > A 100% sized screen set off by 0.2 mm in one direction would still show > about 99.4% of the true image. 99.1% if that 0.2mm error happens to be along the short side of the frame (which would be the case for an error in mirror angle). That 95% is probably a conservative figure; I wouldn't be at all surprised to find that many of my Pentax bodies actually are accurate to 98% levels. But that's still not 100%. To get to 100% (or even to 99.9) gets expensive.
