A new technique for measuring the affective focal length of a thick lens or a compound lens

A new technique, based on the Talbot interferometry and ray tracing, for measuring the affective focal length of a thick lens or a compound lens is proposed. A test lens is positioned between two gratings. First, we adjust the position of the first grating, and take a high contrast moiré pattern. Then we displace the second grating far from the test lens with a known distance. We readjust the position of the first grating and take another high contrast moiré pattern. From the tilt angles of these two moiré patterns and the displaced distance of the second grating, we can evaluate the effective focal length of the test lens.