Quote Originally Posted by BetterSense View Post
...I get 29mm fully open--that's 210mm/29mmm =7.25! That's quite a bit smaller than f/5.6 eh?
Not that much really. The difference is approximately 2/3 stop. Don't use a dial caliper. A steel ruler marked of in millimeters is good enough and easier to get right. 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.