Originally posted by Gooshin ie 55mm giving the same magnification as human vision
I, sincerely, do have to correct you here.
The magnification is a combined function of focal length and view finder magnification. It is by convention only that with a focal length about 50mm, on a 35mm camera, the resulting magnification is about 1. On a camera like the Pentax LX, you can actually change this.
The convention isn't even well founded. It is just a compromise.
The human's field of view approaches 180° and a 12mm focal length would be more appropriate.
On the other hand, the FoV for sharpest vision is a few degrees only and 200mm focal length would be more appropriate.
45° is the compromise adopted by convention, i.e. 50mm for 35mm film. But it is a convention for view finder makers only.