Here's a teaser for tomorrow's installment of Camera Works. In the two photos below we're looking (once again) at my main Nikon lens, front and back. It has a focal length of 50 mm, so it's a "normal" lens.
I know, without measuring it, that the open aperture you see letting light through the middle of the lens is 61/4 mm across at this setting, which is f/8 on the lens's aperture dial:
Tomorrow I'll talk about how I know, why it has that diameter in the first place, and why, for lenses, 22 is smaller than 4.