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 f² = zz ^′ , with the Bessel equation f = ( (L − PP)² − D² ) / 4 ( L − PP ) , with equation f² = y ^′ ( y + y ^′ ) , or with equation f = ( y² − PP² ) / 4PP.