Cameras with active autofocus systems determine the correct focussing distance by projecting beams of energy - usually infrared or light energy - out to the subject.
Some systems measure the reflectivity of the returning infrared and others rely on triangulation. Polaroid even built some instant cameras some decades ago which used ultrasonic sound waves instead of infrared energy - a bizarre system since it couldnt focus through glass.
Such active systems are commonly used on point and shoot cameras. SLRs, by contrast, usually use passive autofocus systems.
cf. autofocus, infrared, light, passive autofocus, point and shoot.
Entry last updated 2002-05-03. Term 19 of 1487.
Previous term: acid-free paper.
Next term: acutance.