active AF

Definition: Active auto-focus: system using a beam of energy to measure distances or range-find for purpose of focusing lens. * Usual arrangement is to use infra-red beam which is swung in an arc over the subject area: the nearest point to reflect light is detected and the angle of the beam measured: from this the distance can be calculated, given the distance between the sensor and the source. * Another design uses an ultra-sound beam: the shift in phase of the reflected ultra-sound is measured to calculate the distance.

Related Terms: coincidence rangefinder, passive AF

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