I have a Canon FD 28mm f/2.8 that I shoot on my entry-level Canon DSLR using an adapter. You have to set the aperture manually, but only every second aperture is marked: between ƒ/2.8 and ƒ/4, for example, there’s an unmarked stop on the aperture ring.
This got me curious: how can I determine the ƒ number of that middle stop? The obvious answer is that it’s ƒ/3.2, and of course the number doesn’t really matter; I should use that stop if it will allow me to make the photograph I’m trying to make.
For the sake of curiosity, though, is there a way to experimentally determine the ƒ number or aperture size when that stop is in use? (I’d prefer a software-based solution, not something that requires equipment beyond the camera and the lens.)