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{{short description|
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| caption2 = One selected green focus point using pinpoint autofocus
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An '''autofocus''' (
Autofocus systems rely on one or more [[sensor|sensors]] to determine correct focus. Some AF systems rely on a single sensor, while others use an array of sensors. Most modern [[SLR camera]]s use [[Through-the-lens metering|through-the-lens]] optical sensors, with a separate [[sensor array]] providing light [[Metering mode|metering]], 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 [[depth of field]] (DOF) at the widest [[aperture]] 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 [[algorithms]] 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.
The data collected from AF sensors is used to control an [[electromechanical]] system that adjusts the focus of the optical system. A variation of autofocus is an ''[[Rangefinder camera#Digital rangefinder|electronic rangefinder]]'', 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.
==History==
Between 1960 and 1973, [[Ernst Leitz GmbH|Leitz]] (Leica)<ref>{{cite web|url=http://www.lfi-online.com/ceemes/page/show/issue_4_09_3 |title=S System: Autofocus – Leica Fotografie International |access-date=2009-05-15 |url-status=dead |archive-url=https://web.archive.org/web/20090621131329/http://www.lfi-online.com/ceemes/page/show/issue_4_09_3 |archive-date=2009-06-21 }}</ref> patented an array of autofocus and corresponding sensor technologies.
The first mass-produced autofocus camera was the [[Konica C35 AF]], a simple [[point and shoot camera|point and shoot]] model released in 1977. The [[Polaroid SX-70]] Sonar OneStep was the first autofocus [[single-lens reflex camera]], released in 1978.
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The [[Pentax ME F|Pentax ME-F]], which used focus sensors in the camera body coupled with a motorized [[photographic lens|lens]], became the first autofocus 35 mm [[Single-lens reflex camera|SLR]] in 1981.
In 1983 [[Nikon]] released the [[Nikon F3#F3AF|F3AF]], their first autofocus camera, which was based on a similar concept to the ME-F.<ref>{{cite web |url=https://imaging.nikon.com/imaging/information/chronicle/history-f3/ |title=Debut of Nikon F3 |website=Nikon |access-date=Nov 10, 2024}}</ref>
The [[Minolta 7000]], released in 1985, was the first SLR with an integrated autofocus system, meaning both the AF sensors and the drive motor were housed in the camera body, as well as an integrated film advance winder — which was to become the standard configuration for SLR cameras from this manufacturer, and also Nikon abandoned their F3AF system and integrated the autofocus-motor and sensors in the body.
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[[Pentax cameras|Pentax]] was the first to introduce focusing distance measurement for SLR cameras with the FA and FA* series lenses from 1991. Their first [[Pentax K-mount|K<sub>AF</sub>-mount]] [[Pentax (lens)|Pentax lenses ]] with AF had been introduced in 1989.<ref>{{cite web|url=http://www.aohc.it/catalogo.php?catalogo=oggetti&id_catalogo=44&filtro%5btipologia%5d=13 |title=Milestones - Asahi Optical Historical Club |access-date=2021-08-29}}</ref>
In 1992, Nikon changed back to lens integrated motors with their AF-I and AF-S range of lenses; today their entry-level DSLRs do not have a focus motor in the body due to the availability of motors in all new developed [[List of Nikon F-mount lenses with integrated autofocus motor|
==Active==
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[[File:Autofocus phase detection.svg|thumb|300px|Phase detection: In each figure (not to scale), the purple skyline 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, and the yellow rectangle represents sensor arrays (one for each aperture). 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 skyline profiles can be used to determine in 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 camera|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 whether the object is in
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 and videography|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 and videography|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>{{cite web|url=http://www.dpreview.com/articles/5402438893/busted-the-myth-of-open-loop-phase-detection-autofocus|title=Busted! The Myth of Open-loop Phase-detection Autofocus}}</ref>
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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.
Some cameras ([[Minolta 7]], [[Canon EOS-1V]], [[Canon EOS-1D|1D]], [[Canon EOS 30D|30D]]/[[Canon EOS 40D|40D]], [[Pentax K-1]], [[Sony DSLR-A700]], [[Sony DSLR-A850|DSLR-A850]], [[Sony DSLR-A900|DSLR-A900]]) also have a few "high-precision" focus points with an additional set of prisms and sensors; they are only active with "[[Lens speed#Fast lenses|fast lenses]]" with certain geometrical [[Aperture|apertures]] (typically [[f-number]] 2.8 and faster). Extended precision comes from the wider effective measurement base of the "range finder"
Some modern sensors (for example one in [[Librem 5]]) include about 2% phase detection pixels on the chip. With suitable software support, that enables phase detection auto focus.
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== Focus motors ==
Modern autofocus is done through one of two mechanisms; either a motor in the camera body and gears in the lens ("screw drive") or through electronic transmission of the drive instruction through contacts in the mount plate to a motor in the lens. Lens-based motors can be of a number of different types, but are often [[ultrasonic motor]]s or [[stepper motor]]s.
Magnets are often used in electromagnetic motors, such as [[Voice coil]] motors (VCMs) and [[Stepper motor]], which move the lens elements to achieve precise focusing.<ref>{{cite web |url=https://www.stanfordmagnets.com/common-applications-for-permanent-magnets.html |title=Industries Applications |website=Stanford Magnets |access-date=Nov 10, 2024}}</ref> The magnetic field interacts with coils to produce motion for adjusting the lens position quickly and accurately based on focus requirements.<ref>{{cite journal |last1=Gong |first1=Junqiang |last2=Luo |first2=Jianbin |year=2024 |title= Rapid and Precise Zoom Lens Design Based on Voice Coil Motors with Tunnel Magnetoresistance Sensors |journal=Applied Science |volume=14 |issue=6 |page=6990 |doi=10.3390/app14166990|doi-access=free }}</ref> Magnets are ideal for this purpose because they enable smooth and rapid adjustments without direct physical contact, enhancing durability and response time.<ref>{{cite patent |country=US |number=9031393B2 |status=patent}}</ref>
Some camera bodies, including all [[Canon EOS]] bodies and the more budget-oriented among [[Nikon DX|Nikon's DX]] models, do not include an autofocus motor and therefore cannot autofocus with lenses that lack an inbuilt motor. Some lenses, such as [[Pentax DA* lenses|Pentax' DA*]] designated models, although normally using an inbuilt motor, can fall back to screwdrive operation when the camera body does not support the necessary contact pins.
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