Simple and accurate method for determining lens focal length

Abstract

A simple and accurate method for determining the focal length of a lens is presented.There is no need to know the distance between lens principal planes and can be performed with easily available and inexpensive hardware. Depending on the test lens (TL) f-number, the error on determining focal length can be in the order of 0.1% of the focal length. We discuss determining focal length with the Newton equation f 2 ¼ zz 0 , with the Bessel equation f ¼ ððL − PPÞ 2 − D 2 Þ∕4ðL − PPÞ, with equation f 2 ¼ y 0 ðy þ y 0 Þ, or with equation f ¼ ðy 2 − PP 2 Þ∕4PP.