I'm trying to accurately measure the focal length of a lens.
I have a complex computerised system lens measuring system that I don't entirely trust (I worked with Hubble) so I want to confirm the focal length - to an accuracy of 1% or better.
- This is a multi-element lens so I don't know precisely where the principle planes are.
- It is pre-focussed at 1m (approx f=0.01m) and I can't re-focus to infinity to test
The plan was to image a precision USAF target at precise stepped distances $\Delta d$ from the lens and measure the image size to get a pixel scale at each increment of distance. Plot image scale against $d+\Delta d$ and iteratively fit an f to minimise variation.
(I don't know absolute $d$ of course because I don't know the principle plane location in the lens)