Who has some advice on formulas or techniques for finding the exact "optical center" of a Canon FD 50mm f/1.4 lens. When I say "optical center" I don't necessarily mean the physical center of the lens length. I've wondered if the following formula is a valid way of finding it:
A Canon FD lens has a "flange to sensor distance" of 42.0mm. I don't know if this is also the "Back Focal Length" (BFL)? Therefore is this true:
[Effective Focal Length] - ["flange to sensor distance"] = "the optical center"
e.g. 50mm - 42mm = 8mm, where the "the optical center" is 8mm from the rear optical surface of the lens.
This formula wouldn't work, or at least it has confusing results when the "Effective Focal Length" is shorter than the "flange to sensor distance".
e.g. 24mm - 42mm = -18mm
Another thought is to just measure for the physical distance between front and rear optical surfaces, i.e. "the exit pupil distance".
["The exit pupil distance"] / 2 = "the physical center"
e.g. A Canon FD 50mm f/1.4 lens has about ~44mm long "exit pupil distance" from front to rear optical surfaces.
So... 44mm / 2 = 22mm, where 22mm is "the physical center" but perhaps not "the optical center".
Can anybody offer an exact way of measuring "THE OPTICAL CENTER"?