Definition: Device for measuring distance between object and observer, usually to focus a lens on the object. * May be based on triangulation: the device deduces the distance by the difference in angles from two viewpoints given a fixed distance between the viewpoints (the base or base-length): this may be measured visually e.g. by making two images coincide or actively, by using infra-red beams sent out at different angles: the distance is deduced from the angle of the reflected beam. * Another type measures distance based on phase differences in the reflection of an ultrasonic beam. * Strictly speaking, passive phase-difference auto-focus systems are not rangefinders.

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