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{{for|the film of the same name|Auto Focus}}
An '''autofocus''' (or '''AF''') [[optical]] system uses a [[sensor]], a [[control system]] and a [[Electric motor|motor]] to [[Focus (optics)|focus]] fully [[wikt:automatic|automatic]] or on a manually selected point or area. An [[Rangefinder_camera#Digital_rangefinder|electronic rangefinder]] has a display instead of the motor; the adjustment of the optical system has to be done manually until indication.
==General==
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Phase detection (PD) is achieved by dividing the incoming light into pairs of images and comparing them. [[Through-the-lens metering|Through the lens]] secondary image registration (TTL SIR) passive phase detection is often used in film and digital [[Single-lens reflex camera|SLR cameras]]. The system uses a [[beam splitter]] (implemented as a small semi-transparent area of the main reflex mirror, coupled with a small secondary mirror) to direct light to an AF sensor at the bottom of the camera. Two micro-lenses capture the light rays coming from the opposite sides of the lens and divert it to the AF sensor, creating a simple [[rangefinder]] with a base within the lens's diameter. The two images are then analysed for similar light intensity patterns (peaks and valleys) and the separation error is calculated in order to find if the object is in [[front focus]] or [[back focus]] position. This gives the direction and an estimate of the required amount of focus ring movement.<ref>[http://www.nikon.com/about/technology/rd/core/software/caf/index.htm Nikon Technology Predictive Focus Tracking System]</ref>
PD AF in a continuously focusing mode (e.g. "AI Servo" for [[Canon EOS|Canon]], "AF-C" for [[Nikon#Digital_single_lens_reflex_cameras|Nikon]], [[Pentax_cameras#Digital|Pentax]] and [[Sony#Photography|Sony]]) is a [[closed-loop control]] process. PD AF in a focus-locking mode (e.g. "One-Shot" for [[Canon EOS|Canon]], "AF-S" for [[Nikon#Digital_single_lens_reflex_cameras|Nikon]] and [[Sony#Photography|Sony]]) is widely believed to be a "one measurement, one movement" [[open-loop control]] process, but focus is confirmed only when the AF sensor sees an in-focus subject. The only apparent differences between the two modes are that a focus-locking mode halts on focus confirmation, and a continuously
Although AF sensors are typically one-dimensional photosensitive strips (only a few pixels high and a few dozen wide), some modern cameras ([[Canon EOS-1V]], [[Canon EOS-1D]], [[Nikon D2X]]) feature TTL area SIR {{citation needed|date=March 2011}} sensors that are rectangular in shape and provide two-dimensional intensity patterns for a finer-grain analysis. Cross-type focus points have a pair of sensors oriented at 90° to one another, although one sensor typically requires a larger aperture to operate than the other.
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