An Autofocus optical system uses a sensor, a control system and a motor to focus fully automatic or on a manually selected point or area. An electronic rangefinder lacks the motor; the adjustment of the optical system has to be done manually until indication.Autofocus systems rely on one or more sensors to determine correct focus. Some AF systems rely on a single sensor, while others use an array of sensors. Most modern SLR cameras use through-the-lens optical AF sensors, with a separate sensor array providing light metering, although the latter can be programmed to prioritise its metering to the same area as one or more of the AF sensors.Through-the-lens optical autofocusing is now often speedier and more precise than what can be achieved manually with an ordinary viewfinder. Autofocus accuracy within 1/3 of the depth of field at the widest aperture of the lens is not uncommon in professional AF SLR cameras.Most multi-sensor AF cameras allow manual selection of the active sensor, and many offer automatic selection of the sensor using algorithms which attempt to discern the location of the subject.

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