Autofocus: Difference between revisions

{{short description|OpticalSensor-controlled system tooptical focus on an automatically or manually selected point or area}}

{{for|the filmsimilarly ofnamed the same namefilm|Auto Focus{{!}}''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]]allyautomatically 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 as [[Passivity (engineering)|active]], [[Passivity (engineering)|passive]] or hybrid types.

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.

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. Canon cameras call this [[AI servo]]; Nikon cameras call it "continuous focus".

The speed of the AF system is highly dependent on the widest aperture offered by the lens at the current focal length. [[F-stop]]s 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 [[teleconverter]]s. {{Citation needed|reason=Looks like original research.|date=October 2016}}

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. At [[photokinaIn 1976]], Leica had presented a camera based on their previous development at [[photokina]], named Correfot, and in 1978 they displayed an SLR camera with fully operational autofocus.

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>

[[Canon (company)|Canon]] decided to discontinue their [[Canon FD lens mount|FD mount]] and switched to the completely electronic [[Canon EF lens mount|EF mount]] with motorised lenses in 1987.

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|the availability of motors in all new developed AF-Lenses]].

There are various ways to measure distance, including [[Ultrasound|ultrasonic]] sound waves and [[infrared]] light. In the first case, sound waves are emitted from the camera, and by measuring the delay in their reflection, distance to the subject is calculated. [[Instant camera|Polaroid]] cameras including the Spectra and [[Polaroid SX-70|SX-70]] were known for successfully applying this system. In the latter case, infrared light is usually used to [[triangulation|triangulate]] the distance to the subject. Compact cameras including the [[Nikon]] [[Nikon Ti cameras|35TiQD and 28TiQD]], the [[Canon AF35M]], and the [[Contax T|Contax T2 and T3]], as well as early video cameras, used this system. A newer approach included in some consumer electronic devices, like mobile phones, is based on the [[Time of flight|time-of-flight]] principle, which involves shining a laser or LED light to the subject and calculating the distance based on the time it takes for the light to travel to the subject and back. This technique is sometimes called ''laser autofocus'', and is present in many mobile phone models from several vendors. It is also present in industrial and medical<ref>{{cite journal |last1=Fricke |first1=Dierk |last2=Denker |first2=Evgeniia |last3=Heratizadeh |first3=Annice |last4=Werfel |first4=Thomas |last5=Wollweber |first5=Merve |last6=Roth |first6=Bernhard |title=Non-Contact Dermatoscope with Ultra-Bright Light Source and Liquid Lens-Based Autofocus Function |journal=Applied Sciences |date=28 May 2019 |volume=9 |issue=11 |pages=2177 |doi=10.3390/app9112177|doi-access=free }}</ref> devices.

[[File:Autofocus phase detection.svg|thumb|300px|Phase detection: In each figure (not to scale), the area within the purple circleskyline 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), and the graph represents the intensity profile as seen by each sensor array. 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 [[front focus]] or [[back focus]] position. This gives the direction and an estimate of the required amount of focus-ring movement.<ref>{{cite web|url=http://www.nikon.com/about/technology/rd/core/software/caf/index.htm|title=Nikon - Technology - Predictive Focus Tracking System|access-date=2013-11-12|archive-url=https://web.archive.org/web/20131112195552/http://www.nikon.com/about/technology/rd/core/software/caf/index.htm|archive-date=2013-11-12|url-status=dead}}</ref>

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".

Contrast-detection autofocus is achieved by measuring [[Contrastcontrast (vision)]] within a sensor field [[Through-the-lens metering|through the lens]]. The intensity difference between adjacent pixels of the sensor naturally increases with correct image focus. The optical system can thereby be adjusted until the maximal contrast is detected. In this method, AF does not involve actual distance measurement at all. This creates significant challenges when [[Video tracking|tracking moving subjects]], since a loss of contrast gives no indication of the direction of motion towards or away from the camera.

Similar [[Stroboscopic effect|stroboscopic]] flashing is sometimes used to reduce the [[red-eye effect]], but this is only intended to constrict the subject's eye pupils before the shot.

Some external [[flash gun]]s have integrated autofocus assist lamps that replace the stroboscopic on-camera flash. Many of them are red and less obtrusive. Another way to assist contrast based AF systems in low light is to beam a laser pattern on to the subject. The laser method is commercially called Hologram AF Laser and was used in [[Cyber-shot|Sony CyberShot]] cameras around the year 2003, including Sony's F707, [[Sony Cyber-shot DSC-F717|F717]] and [[Sony Cyber-shot DSC-F828|F828]] models.

A rare example of an early hybrid system is the combination of an active [[Infrared|IR]] or ultrasonic auto-focus system with a passive phase-detection system. An IR or ultrasonic system based on reflection will work regardless of the light conditions, but can be easily fooled by obstacles like window glasses, and the accuracy is typically restricted to a rather limited number of steps. Phase-detection autofocus "sees" through window glasses without problems and is much more accurate, but it does not work in low-light conditions or on surfaces without contrasts or with repeating patterns.

A very common example of combined usage is the phase-detection auto-focus system used in [[Single-lens reflex camera|single-lens reflex cameras]] since the 1985s. The passive phase-detection auto-focus needs some contrast to work with, making it difficult to use in low-light scenarios or on even surfaces. An [[AF illuminator]] will illuminate the scene and project contrast patterns onto even surfaces, so that phase-detection auto-focus can work under these conditions as well.

A newer form of a hybrid system is the combination of passive phase-detection auto-focus and passive contrast auto-focus, sometimes assisted by active methods, as both methods need some visible contrast to work with. Under their operational conditions, phase-detection auto-focusing is very fast, since the measurement method provides both information, the amount of offset and the direction, so that the focusing motor can move the lens right into (or close to) focus without additional measurements. Additional measurements on the fly, however, can improve accuracy or help keep track of moving objects. However, the accuracy of phase-detection auto-focus depends on its effective measurement basis. If the measurement basis is large, measurements are very accurate, but can only work with lenses with a large geometrical [[aperture]] (e.g. 1:2.8 or larger). Even with high contrasty objects, phase-detection AF cannot work at all with lenses slower than its effective measurement basis. In order to work with most lenses, the effective measurement basis is typically set to between 1:5.6 and 1:6.7, so that AF continues to work with slow lenses (at least for as long as they are not stopped down). This, however, reduces the intrinsical accuracy of the autofocus system, even if fast lenses are used. Since the effective measurement basis is an optical property of the actual implementation, it cannot be changed easily. Very few cameras provide multi-PD-AF systems with several switchable measurement bases depending on the lens used in order to allow normal auto-focusing with most lenses, and more accurate focusing with fast lenses.

Contrast AF does not have this inheritinherent design limitation on accuracy as it only needs a minimal object contrast to work with. Once this is available, it can work with high accuracy regardless of the speed of a lens; in fact, for as long as this condition is met, it can even work with the lens stopped down. Also, since contrast AF continues to work in stopped-down mode rather than only in open-aperture mode, it is immune to [[Spherical aberration|aperture-based focus shift errors]] phase-detection AF systems suffer since they cannot work in stopped-down mode. Thereby, contrast AF makes arbitrary fine-focus adjustments by the user unnecessary. Also, contrast AF is immune to focusing errors due to surfaces with repeating patterns and they can work over the whole frame, not just near the center of the frame, as phase-detection AF does. The down-side, however, is that contrast AF is a closed-loop iterative process of shifting the focus back and forth in rapid succession. Compared to phase-detection AF, contrast AF is slow, since the speed of the focus iteration process is mechanically limited and this measurement method does not provide any directional information. Combining both measurement methods, the phase-detection AF can assist a contrast AF system to be fast and accurate at the same time, to compensate aperture-based focus-shift errors, and to continue to work with lenses stopped down, as, for example, in stopped-down measuring or video mode.

Recent developments towards [[Mirrorless camera|mirrorless cameras]] seek to integrate the phase-detection AF sensors into the image sensor itself. Typically, these phase-detection sensors are not as accurate as the more sophisticated stand-alone sensors, but since the fine focusing is now carried out through contrast focusing, the phase-detection AF sensors are only need to provide coarse directional information in order to speed up the contrast auto-focusing process.

In July, 2010, [[Fujifilm]] announced a compact camera, the F300EXR, which included a hybrid autofocus system consisting of both phase-detection and contrast-based elements. The sensors implementing the phase-detection AF in this camera are integrated into the camera's [[Super CCD]] EXR.<ref>[http://www.fujifilmusa.com/press/news/display_news?newsID=879875 Fujifilm Launches Powerhouse 15X Long Zoom Point and shoot Digital Camera: The FinePix F300EXR] {{Webarchive|url=https://web.archive.org/web/20100727174907/http://www.fujifilmusa.com/press/news/display_news?newsID=879875 |date=2010-07-27 }}, Fujifilm, USA</ref> Currently it is used by [[Fujifilm FinePix]] Series,<ref>{{cite web |url=http://www.dpreview.com/news/2013/01/07/Fujifilm-finepix-hs50-exr-hs35 |title=Fujifilm launches FinePix HS50EXR and HS35EXR high-end superzooms |access-date=June 8, 2013}}</ref> [[Fujifilm X100|Fujifilm X100S]], [[Ricoh]], [[Nikon 1 series]], [[Canon EOS 650D|Canon EOS 650D/Rebel T4i]] and [[Samsung NX300]].

File:My Canon AF35M (4307694589).jpg|Active autofocus system via infrared - [[Canon AF35M]] (1979)

File:Pentax ME-F autofocus.jpg|Early passive autofocus system integrated in the lens with [[Pentax ME F|Pentax ME-F]] (1981)

File:D4-85 1.4.JPG|Modern (2014) autofocus single lens reflex camera - [[Nikon D4]]

The first [[Single-lens reflex camera|SLR]] to implement trap focusing was the [[Yashica 230 AF]]. Trap focus is also possible on some Pentax (e.g. [[Pentax K-x|K-x]] and [[Pentax K-5|K-5]]), [[Nikon]], and [[Canon EOS]] cameras. The [[Canon EOS-1D|EOS 1D]] can do it using software on an attached computer, whereas cameras like the [[Canon EOS 40D|EOS 40D]] and [[Canon EOS 7D|7D]] have a custom function (III-1 and III-4 respectively) which can stop the camera trying to focus after it fails.{{citation needed|date=September 2013|reason=The description for the 40D and 7D does not sound as if they would actually support focus trapping. Please provide references.}} On EOS cameras without genuine trap focus, a hack called "almost trap focus" can be used, which achieves some of the effects of trap focus.<ref>[http://eosdoc.com/manuals/?q=ATrapFocus EOS Documentation Project: Almost Trap Focus] {{webarchive |url=https://web.archive.org/web/20100818143041/http://eosdoc.com/manuals/?q=ATrapFocus |date=August 18, 2010 }}, by Julian Loke</ref> By using the custom firmware [[DIGIC#Magic Lantern|Magic Lantern]], some Canon DSLRs can perform trap focus.