Quote Originally Posted by fschifano View Post
[...] Measure the apparent diameter of the aperture through the entrance pupil. If the hole isn't circular measure from corner to corner and side to side and average the two. It's close enough.
means that you shouldn't

Quote Originally Posted by fschifano View Post
Oh yeah, measure the aperture from the front, not the back. If you can remove the front cells to measure it, even better. [...]