Testing aperture accuracy

[Larry Huppert]

I've recently had my shutters tested, and found the results very useful in terms of setting more accurate exposures. It occurred to me that testing the shutter is only one part of the equation. The other part is to know how accurate your aperture settings are as well.

Does anyone know of a relatively simple methodology for testing apertures? Expo sing a bunch of transparency film would certainly be one way, but I was looking for something where you might be able to use an incident light meter. I imagine an "absolute" determination of aperture might be near impossible to determine, but at least knowing the relative relationships between different setting on a s ingle lens, and between different lenses would be useful.

[Struan Gray]

Focus the lens on infinity, and then find a diffuse light source that fills its field of view (a light table under a copy stand works well - the table surface doesn't need to be in focus). Then take meter readings off the centre of the ground glass at various apertures. Provided your light source is stable (check it with the meter), you can compare different lenses and different apertures more than well enough for practical photographic purposes.

[Alan Gibson]

Shutters are complicated things that go out of alignment. Apertures are simpler things, and generally don't. If the blades close symmetrically, it's probably OK.

You can measure the aperure by placing a ruler across the front of the lens. Measure the apparent diameter of the hole (in millimetres). Divide this into the focal length (also in mm), and that is the f- number.