Sometimes I need to refer to these aperture scales when preparing material. And sometimes lenses are just off a little bit (or a lot). Also, cameras round f-stops and exposure values, and this leads to inconsistent exposure values with some cameras.
For example, when a camera displays ƒ/5.6, is it ƒ/5.6 or ƒ/5.657 (the actual one-stop difference from ƒ/4)? And is ƒ/1.2 actually ƒ/1.19 or ƒ/1.122?
Of course, in the latter case of a bright aperture (ƒ/1.2), the T-stop matters a whole lot more. It’s not much of a difference but video shooters doing precision work presumably care. Further complicating matters, the distribution of light across the frame can change even in the center even at the same exposure value (EV), so all sorts of differences can accrue to some confusion in total. It’s a regular headache when preparing aperture series for presentation.
One also has to wonder whether 1/13 second is twice 1/25 second: is it really 1/13 second, or actually 2/25 second and displayed as 1/13?
I see minor variations all the time with aperture series when there ought to be none, so I suspect that cameras by and large either are doing it wrong (rounding the actual exposure not just rounding for display), or else have errors for aperture or shutter speed, or both.
ƒ = √(2^AV) where AV is the aperture value.
This table of f-stops / apertures shows half stop aperture series, 1/3 (third) stop aperture series and whole stop aperture series. Aperture / f-stop rounding is common and especially at wider apertures the rounding is less important than the T-stop. Moreoever it cannot be assumed that the lens diaphragm delivers precisely the value one expected: some lens diaphragms are not perfectly symmetrical and there is the precise diameter as well.