An instrument has been prototyped for the purpose of determining the effective focal length of a spherical lens. The instrument employs a two-dimensional position-sensitive detector, two diode laser modules, a microcontroller, and an LCD readout. The system is designed to be automated, be immune to modest misalignment of the test lens, require no mechanical adjustments or image processing, and be accurate for spherical lenses with positive or negative focal lengths in the range of 25 mm<|f|<1,000 mm, and lens diameters no less than 25 mm. The user only needs to insert the test lens. By incorporating a Fourier lens in front of the detector and averaging the signals from the two beams that enter the system parallel and symmetrical with respect to the optic axis, the effect of a modest longitudinal, lateral, or tilt misalignment may be negated.