Active
Active AF systems measure distance to the subject independently of the optical system, and subsequently adjust the optical system for correct focus.
There are various ways to measure distance, including ultrasonic sound waves and infrared light. In the first case, sound waves are emitted from the camera, and by measuring the delay in their reflection, distance to the subject is calculated. Polaroid cameras including the Spectra and SX-70 were known for successfully applying this system. In the latter case, infrared light is usually used to triangulate the distance to the subject. Compact cameras including the Nikon 35TiQD and 28TiQD, the Canon AF35M, and the Contax T2 and T3, as well as early video cameras, used this system.
An exception to the two-step approach is the mechanical autofocus provided in some enlargers, which adjust the lens directly.
Read more about this topic: Autofocus
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