Originally posted by Canada_Rockies Not when the lens is in the mount.
Sorry, you right as you were talking about a "thin lens" (element) at the mount position. I just read "lens" and assumed you meant a camera lens that can be focused.
Originally posted by Canada_Rockies I know this theory about the "normal" lens being the diagonal of the format, but my personal take is that the lens + finder optics should make the image appear the same size as the other eye sees.
I see the merit in your personal take but note that than the definition of a "normal" lens depends on the viewfinder magnification. The later varies a lot between cameras. What really counts is the perspective captured in the final image, not the temporary view through the viewfinder. If one can make the latter coincide with what your other eye sees while taking the shot, all the better, but this is a question of getting the viewfinder magnification right, not of adapting a focal length to an arbitrary viewfinder magnification.
Originally posted by Canada_Rockies There is a third theory that the lens should approximate the field of view...
The AOV of the human eye is very difficult to define (irregular shape, both eyes or just one?, area of sharp vision or peripheral vision included?, individual differences in broadness of vision) that's why I think that trying to define a "normal" lens in terms of the eye's AOV is futile.
Originally posted by rhodopsin Of course, that is true only from the same taking position.
When you don't change the position, the perspective distortion doesn't change. Different focal lengths don't influence perspective. Only the change in position that makes them capture the same image does.