This depends on the video resolution of the sensor and its viewing angle. By increasing the video resolution the software will detect faces at a further distance, but it will require more CPU.

Typically it manages to detect faces up to 13 meters (40 feet) in a 65° field of view with standard Full HD webcams, or up to 25m (75 feet) by using a 5MP resolution with an optical zoom providing 45° field of view.

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