Various instruments to measure the refractive state of the eye have been built based on a servosystem concept. This paper describes the optical system of the Dioptron automatic objective refractor. This autorefractor measures the modulation of a grating image formed on the retina while being controlled by a digital computer. The optical subsystems include the image projecting system, the image detecting system, the patient fixation target, and the patient alignment system. Accuracy is obtained by controlling the patient's accommodation and eliminating "instrument" myopia. The overall system is designed to be compact while maintaining very good signal-to-noise ratio and minimum operator controls.