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I'm looking for suggestions building optical setup to measure changes in Effective Focal Length caused by a temperature gradient in the lens area. Currently, I'm thinking of placing a mirror at the image plane (instead of the sensor) and using an autocollimator to determine how much I'm out of focus and then convert it focal shift using some basic Ray transfer matrix analysis.

Problem is that I'm determining how much the autocollimator is out of focus based on my eye and thus I can easily miss 2 mm which translate to ~2um in the focal shift (assuming Effective Focal length = 8mm and F# = 1.7).

Any Ideas?

BR,

Yonatan Wiegner

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Focal length variations in your lens will directly affect the magnification of your optical system. If you have a reference object with known dimensions, you can figure out from its image the focal length of your lens.

Better yet, even if you don't know the dimensions of the object, you can measure the variations in magnifications in a relative manner (i.e the image is now X% bigger than what it was before).

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