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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]] on an [[wikt:automatic|automatic]]ally or 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. Autofocus methods are distinguished by their type as being either [[Passivity (engineering)|active]], [[Passivity (engineering)|passive]] or hybrid variants.
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[[File:Autofocus phase detection.svg|thumb|upright|''Phase detection''<br /><br />In each figure (not to scale), the purple circle represents the object to be focused on, the red and green lines represent light rays passing through apertures at the opposite sides of the lens, the yellow rectangle represents sensor arrays (one for each aperture), and the graph represents the intensity profile as seen by each sensor array.<br /><br />Figures 1 to 4 represent conditions where the lens is focused (1) too near, (2) correctly, (3) too far and (4) much too far. The phase difference between the two profiles can be used to determine which direction and how much to move the lens to achieve optimal focus.]]
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>
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 focusing mode has predictive elements to work with moving targets, which suggests they are the same closed-loop process.<ref>
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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===AI servo===
AI servo is an auto focus mode found on [[Canon Inc.|Canon]] SLR cameras, although the same principle is used with [[Nikon]] and some [[Pentax]] cameras, known there as "continuous focus" (AF-C).<ref>
== Focus motors ==
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