"Swinging the back of course would move the subject off-center." - Huh?
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"Swinging the back of course would move the subject off-center." - Huh?
Try contacting the people at Zig Align. They make alignment products for enlargers, view cameras, copystands, etc. I use their enlarger system--it's improved the sharpness of my prints so much I won't print without it anymore.
http://www.zig-align.com/index.html
Maybe I'm misunderstanding your question, but it sounds like you want to take straight on shots with no perspective correction. Rather than measure your film plane to subject distance on each side, couldn't you just ensure your lens and film planes are parallell and centered, and then just move the camera to the desired location for framing?
If one leg of the chair is closer to the camera it will naturally appear larger, using a longer lens with a greater working distance would also help diminish this effect.
I know I'm probably missing something.
You may want to look at a laser rangefinder. They are not that expensive anymore and will have an accuracy of perhaps 1/4 inch. It would certainly solve the problem of your 12 foot tape measure not being long enough or falling off when measuring. Look at the better hardware stores that cater to the contractors.
OK, I re-read your post, you seem to be asking how to get the camera precisely centered in front of a seated (or similar) subject. This can be done more simply without measuring to the edges of your camera back. From a geometric point of view you want to make your camera the top of an isosoles triangle (I was a math major a couple years ago, but I'm sure I'm misspelling that) with the base being the plane of your subject that you want to be parallell to, and then center the subject in the frame. From a measurement point of view, you might get more accuracy if you expand the base of the triangle for the purpose of your measurement (e.g. place a yardstick or longer straightedge centered on the center of your subject, parallell with your subject plane and take your measurements from the ends).
Doesn't help with you needing a longer tape measure though. I've never needed this much accuracy, I just eyeball it and compose based on what I see on the glass.
During set up, if you are able to place a straight edge in the subject position parallel to what will be an axis through the width of the subject, then you can use a backpacking style compass. I'm thinking of the kind made by Silva that allows you to rotate the dial to get the engraved arrow exactly under the floating magnetic needle. You take the bearing from the straight edge and then make sure that a measurement taken at the film plane is the same. You should easily be able to be accurate within one degree. njb
Perhaps I don't understand exactly what you are asking. If you are simply interested in making sure the camera back is horizontally parallel to a wall or other sort of background (vertical you should be able to take care of with a level), then just make sure that the horizontal lines in the background are perfectly horizontal and parallel on the ground glass. If this is the case, you must be parallel to the background (within visual tolerances, at least). If your subject is then off-center, you need to move the camera and bring the back parallel to the background again. If there are no horizontal lines in you background, either make some temporary ones (hang a meter stick or draw some lines) or ignore it, since it makes no visual difference anyway. In the latter case, just make your best composition on the gg. Hope this helps a bit. ;^D)
Do you have grid lines on your groundglass? If you have a plain groundglass, you can pencil them in on the matte side. If you have a glass with a fresnel on the matte side, you can draw them in on the smooth side with a felt-tip pen that uses water soluble ink.
Put a mirror at the subject location. With all of your rises, shifts and tilts zeroed out, move your camera around, and when you can see the reflection of your camera lens centered on your groundglass, then your camera's axis will be perpendicular to the mirror. Does that make sense?