Autofocus: Difference between revisions

Through-the-lens optical autofocusing is now oftenusually speedier and more precise than manual can be achieved manuallyfocus with an ordinary viewfinder, although more precise manual focus can be achieved with special accessories such as [[focusing magnifier]]smagnifiers. Autofocus accuracy within 1/3 of the [[depth of field]] (DOF) at the widest [[aperture]] of the lens is not uncommoncommon 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 data, and keep focus on the subject — afocus — a function used mainly in sports and other action photography; on. Canon cameras call this is known as [[AI servo]], while on; Nikon cameras call it is known as "continuous focus".

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 ''electronic [[Rangefinder camera#Digital rangefinder|electronic rangefinder]]'', a system 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 maximumwidest aperture offered by the lens at the current focal length. [[F-stop]]s of around {{f/}}2 to {{f/}}2.8 are generally considered optimalbest in terms offor 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}}

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|upright300px|''Phase detection''<br: /><br />In each figure (not to scale), 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.<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 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. Secondary image registration (SIR), [[Through-the-lens metering|through 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 ifwhether 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>

PD AF in a continuously focussingfocusing mode (e.g. "AI Servo" for [[Canon EOS|Canon]], "AF-C" for [[Nikon#Digital_single_lens_reflex_camerasDigital single lens reflex cameras|Nikon]], [[Pentax_camerasPentax 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_camerasDigital 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 focussingfocusing 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>

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 (Canon EOS-1V, Canon EOS-1D, [[Canon EOS 30D]]/[[Canon EOS 40D|40D]]) 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]]' of certain [[F-number|focal ratio]]. Extended precision comes from the increased diameter of such lenses, so the base of the 'range finder' can be wider.

7331, 74 -32 – Pair of aperturesregions;

Contrast -detection autofocus is achieved by measuring [[contrast (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 maximummaximal contrast is detected. In this method, AF does not involve actual distance measurement at all and is generally slower than phase detection systems, especially when operating under dim light. Furthermore, as the autofocus system does not calculate whether the subject is in front focus or back focus, focus tracking is not feasible. As it does not use a separate sensor, however, contrast-detect autofocus can be more flexible (as it is implemented in software) and potentially more accurate. This iscreates asignificant commonchallenges method inwhen [[videoVideo cameras]] and [[point-and-shoot cameratracking|digitaltracking camera]]smoving that lack [[Shutter (photography)|shuttersubjects]]s, and reflex mirrors. Most [[digital single-lens reflex camera|DSLR]]s use this method (orsince a hybridloss of both contrast andgives phaseno detectionindication autofocus) when focusing in their [[live-preview digital camera|live-view]] modes. [[Mirrorless interchangeable-lens camera]]s, including [[Micro Four Thirds]], usually use contrast measurement autofocus (phase detect is available on some models, such asof the Nikondirection 1,of ormotion via a lens adapter). Their manufacturers claim performance comparabletowards or evenaway faster than phase detection systems,<ref>[http://www.olympusamerica.com/oai_HeadlinesDetails.asp?pressNo=855 Olympus® Launchesfrom the PEN® E-P3: Are You PEN Ready?], Olympus</ref><ref>[http://panasonic.net/avc/lumix/systemcamera/gms/g3/af.html The World's Fastest Level of Light Speed AF], Panasonic</ref> though it is unclear under what conditions (e.g., dim light) these claims hold truecamera.

The assist light (also known as AF illuminator) "activates" passive autofocus systems in low-light and low-[[Contrast (vision)|contrast]] situations in some cameras ([[Nikon_Corporation#Digital_single_lens_reflex_cameras|Nikon]]). The lamp projects visible or [[infrared|IR]] light onto the subject, which the camera's autofocus system uses to achieve focus.

Many cameras thatand donearly notall have[[camera phone]]s{{efn|Counter-examples are the [[Nokia Lumia 1020]], the [[Samsung Galaxy S4 Zoom]] and the [[Samsung Galaxy K Zoom]].}} lack a dedicated autofocus assist lamp. instead Instead, they use their built-in flash, illuminating the subject with stroboscopic bursts of light. The strobe burstsThis aidaids the autofocus system in the same fashion as a dedicated assist light, but havehas the disadvantage of startling or annoying living subjectspeople. Another disadvantage is that if the camera uses flash focus assist and is set to an operation mode that overrides the flash, it may also disable the focus assist. and Thus, autofocus may fail to acquire the subject.

Similar [[Stroboscopic effect|stroboscopic]] flashing is sometimesometimes used to reduce the [[red-eye effect]], but this method is only intended to constrict the subject's eye pupils prior tobefore the actual shot being taken, and thus reduce retinal reflections.

In some cases,Some external [[flash gunsgun]]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 CybershotCyberShot]] cameras around the year 2003, including Sony's F707, [[Sony Cyber-shot DSC-F717|F717]] and [[Sony Cyber-shot DSC-F828|F828]] models.

Active systems may also fail to focus a subject that is very close to the camera (''e.g.'', [[macro photography]]).

A method variously referred to as ''trap focus'', ''focus trap'', or ''catch-in-focus'' uses autofocus to take a shot when a subject moves into the focal plane (at the relevant focal point); this can be used to get a focused shot of a rapidly moving object, particularly in sports or [[wildlife photography]], or alternatively to set a "trap" so that a shot can automatically be taken without a person present. This is done by using AF to ''detect'' but not ''set'' focus&nbsp;– using manual focus to set focus (or switching to manual after focus has been set) but then using ''[[focus priority]]'' to detect focus and only release the shutter when an object is in focus. The technique works by choosing the focus adjustment (turning AF off), then setting the shooting mode to "Single" (AF-S), or more specifically focus priority, then depressing the shutter&nbsp;– when the subject moves into focus, the AF detects this (though it does not change the focus), and a shot is taken.<ref>[http://www.shutterstock.com/tips-tricks/Trap-Focus-for-Nikon-Users-100.html Trap Focus for Nikon Users], by Kenneth William Caleno, January 28, 2009</ref><ref>[http://www.kenrockwell.com/tech/sports.htm How to shoot sport], Ken Rockwell, 2006</ref><ref>[http://photographyrulez.blogspot.com/2009/04/focus-trap-or-catch-in-focus.html Focus Trap or Catch In Focus], April 4, 2009</ref>

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_LanternMagic Lantern|Magic Lantern]], some Canon DSLRs can perform trap focus.

===AI servoServo===

AI servoServo is an [[auto focus]]autofocus mode found on [[Canon Inc.|Canon]] SLR cameras, althoughand thein sameother principlebrands issuch used withas [[Nikon]], [[Sony]], and some [[Pentax]] cameras, knownunder therethe asname "continuous focus" (AF-C).<ref>[{{cite web|url=http://www.dpreview.com/learn/?/Glossary/Camera_System/af_servo_01.htm|title=Articles AFtagged Servo: Camera System: Glossary: Learn"learn": Digital Photography Review<!-- Bot generated title -->]}}</ref> Also referred to as ''focus tracking'', it is used to track a subject as it moves around the frame, or toward and away from the camera. When in use, the lens will constantly maintain its focus on the subject, hence it is commonly used for [[Sports photography|sports and action photography]]. AI refers to [[artificial intelligence]]: algorithms that constantly predict where a subject is about to be based on its speed and acceleration data from the autofocus sensor.

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