Hi, Dean,
Let's also consider the mirror aspect...this camera uses a mirror that's coated on the upside, same as all SLR's. These things are mighty expensive, so what compensation do I make when using a cheaper mirror that is coated on the underside like the one on your bathroom wall?
Say it's 2mm thick, does that mean I must space the film plane 2mm farther back than normal, or if it's on a 45 degree angle is that 1mm?
If you could confirm the situation, I can show how to make the calculation. (I came into the story late.)
I suspect we basically saying that we have a reflex camera designed to use a front-surface mirror, but we don't have the original mirror, and are going to put a rear-surface mirror in it (for economy or whatever), keeping the mirror and focusng screen locations the same? Then we need to know how to shift the film plane to compensate for the disruption caused by the mirror change?
I'll proceed as if that were so.
If we put a transparent glass plate of thickness
t into the path of an image being focused by a lens (the plate being perpendicular to the lens axis), the point of focus is moved farther away from the lens by the amount n-1(
t), where n is the index of refraction of the glass. For typical kinds of glass, the index of refraction might be about 1.4. Thus for a 1 mm thick plate, the point of focus is "pushed back" by about 0.4 mm.
Now in the situation I am assuming, with regard to focus via the mirror on the ground glass, the ray bundles travel twice through the glass, and at an angle of 45 degrees to boot. Thus the effective distance traveled through the 1-mm thick plate is about 2.8 mm (1.4 mm going, 1.4 mm coming.)
That means that, if the camera is focused so that with the mirror up, the image would fall on the standard film plane, with the mirror in place it will fall about 2.8 mm
beyond the ground glass.
But of course we will focus so the image falls on the ground glass. This means that when focus on the ground glass is achieved, the lens will be farther from the mirror (forward) by about 2.8 mm (assuming we are speaking of an object at some reasonable distance) than with a front-surface mirror.
Then, when we flip the mirror up to take the exposure, the image will fall about 2.8 mm
in front of the standard film plane.
Thus we need to move the film plane
forward (toward the lens) about 2.8 mm to compensate for the different mirror situation.
These numbers would all be doubled for a 2-mm thick mirror, if course.