A system and method that measures an optical focus of a distant optical imaging system (EYE), in particular the ocular accommodation of a distant human subject. A luminous pattern of light (P1, A1) is projected by a projector (P) in focus (A2) at a known focal plane (FPL1) in front of the distant optical imaging system (EYE), and an image of the reflection of the pattern (A3) on a sensor surface of the distant optical imaging system (EYE), for instance the retina of an eye, is recorded by a camera (CAM) having an optical axis (AXCAM) coinciding at least partly with or situated close to the optical axis (AXP) of the projection device (P). The sharpness of the luminous pattern (A3) reflected from the sensor surface (retina) is determined.