This calculator helps you find the missing value of a lens: focal length, f-stop, or aperture diameter, by entering the other two. It’s especially useful for vintage projection or photographic lenses that don’t have marked aperture values.

To estimate the aperture diameter, shine a light from behind the lens and measure the apparent size of the aperture (called the entrance pupil) from the front, you’re essentially measuring ‘the air’. This gives a good approximation for use in the calculator.

In some cases, you can also use the diameter of the front element as a rough guide, but this usually results in an aperture that appears slightly brighter than the actual value.

I’ve made three other tools for vintage lenses, the 35mm Equivalent Lens Calculator, Vintage Projection Lens Coverage Tool and the Vintage Lens Year Calculator.