Autofocus

An autofocus (AF) <a href="/facts/Optical/p0vq4S1T">optical</a> system uses a <a href="/facts/Sensor/RN8SXbjs">sensor</a>, a <a href="/facts/Control_system/8CEjIDlS">control system</a> and a <a href="/facts/Electric_motor/ArD7BGDa">motor</a> to <a href="/facts/Focus_(optics)/QuRKfhg6">focus</a> on an automatically or manually selected point or area. An <a href="/facts/Rangefinder_camera/75bnR2ld">electronic rangefinder</a> has a display instead of the motor; the adjustment of the optical system has to be done manually until indication. Autofocus methods are distinguished as <a href="/facts/Passivity_(engineering)/7oDeoqd3">active</a>, <a href="/facts/Passivity_(engineering)/7oDeoqd3">passive</a> or hybrid types.
Autofocus systems rely on one or more <a href="/facts/Sensor/RN8SXbjs">sensors</a> to determine correct focus. Some AF systems rely on a single sensor, while others use an array of sensors. Most modern <a href="/facts/SLR_camera/oTTVInva">SLR cameras</a> use <a href="/facts/Through-the-lens_metering/lhGQcN9N">through-the-lens</a> optical sensors, with a separate <a href="/facts/Sensor_array/LEuhQDY5">sensor array</a> providing light <a href="/facts/Metering_mode/dMRaYz63">metering</a>, although the latter can be programmed to prioritize its metering to the same area as one or more of the AF sensors.
Through-the-lens optical autofocusing is usually speedier and more precise than manual focus with an ordinary viewfinder, although more precise manual focus can be achieved with special accessories such as focusing magnifiers. Autofocus accuracy within 1/3 of the <a href="/facts/Depth_of_field/xWo3J8b0">depth of field</a> (DOF) at the widest <a href="/facts/Aperture/8PV11GQm">aperture</a> of the lens is common 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 <a href="/facts/Algorithms/fnl5NmRt">algorithms</a> which attempt to discern the location of the subject. Some AF cameras are able to detect whether the subject is moving towards or away from the camera, including speed and acceleration, and keep focus — a function used mainly in sports and other action photography. Canon cameras call this <a href="/facts/AI_servo/1B0tQWg2">AI servo</a>; Nikon cameras call it "continuous focus".
The data collected from AF sensors is used to control an <a href="/facts/Electromechanical/0D5gkXko">electromechanical</a> system that adjusts the focus of the optical system. A variation of autofocus is an <a href="/facts/Rangefinder_camera/75bnR2ld">electronic rangefinder</a>, in which focus data are provided to the operator, but adjustment of the optical system is still performed manually. 
The speed of the AF system is highly dependent on the widest aperture offered by the lens at the current focal length. <a href="/facts/F-stop/QcdW0077">F-stops</a> of around f/2 to f/2.8 are generally considered best for focusing speed and accuracy. Faster lenses than this (e.g.: f/1.4 or f/1.8) typically have very low depth of field, meaning that it takes longer to achieve correct focus, despite the increased amount of light. Most consumer camera systems will only autofocus reliably with lenses that have a widest aperture of at least f/5.6, whilst professional models can often cope with a widest aperture of f/8, which is particularly useful for lenses used in conjunction with <a href="/facts/Teleconverter/se4bM1Gh">teleconverters</a>.

Autofocus open-in-new

Autofocus