840:(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 inherent 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
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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.
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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
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298:, 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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649:. 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
498:(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
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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 – when the subject moves into focus,
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Phase detection system: 7 – Optical system for focus detection; 8 – Image sensor; 30 – Plane of the vicinity of the exit pupil of the optical system for photography; 31, 32 – Pair of regions; 70 – Window; 71 – Visual field mask; 72 – Condenser lens; 73, 74 – Pair of apertures; 75 – Aperture mask; 76,
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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
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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.
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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
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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
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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
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lack a dedicated autofocus assist lamp. Instead, they use their built-in flash, illuminating the subject with bursts of light. This aids the autofocus system in the same fashion as a dedicated assist light, but has the disadvantage of startling or annoying people. Another disadvantage is that if
566:) feature TTL area SIR 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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Passive systems may not find focus when the contrast is low, notably on large single-colored surfaces (walls, blue sky, etc.) or in low-light conditions. Passive systems are dependent on a certain degree of illumination to the subject (whether natural or otherwise), while active systems may focus
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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
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Passive AF systems determine correct focus by performing passive analysis of the image that is entering the optical system. They generally do not direct any energy, such as ultrasonic sound or infrared light waves, toward the subject. (However, an autofocus assist beam of usually infrared light is
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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
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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,
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Active systems will typically not focus through windows, since sound waves and infrared light are reflected by the glass. With passive systems this will generally not be a problem, unless the window is stained. Accuracy of active autofocus systems is often considerably less than that of passive
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Contrast detection places different constraints on lens design when compared with phase detection. While phase detection requires the lens to move its focus point quickly and directly to a new position, contrast-detection autofocus instead employs lenses that can quickly sweep through the focal
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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
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have a custom function (III-1 and III-4 respectively) which can stop the camera trying to focus after it fails. 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. By using the custom firmware
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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.
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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
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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
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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
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correctly even in total darkness when necessary. Some cameras and external flash units have a special low-level illumination mode (usually orange/red light) which can be activated during auto-focus operation to allow the camera to focus.
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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
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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
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typically used contrast-measurement autofocus, although phase detection has become the norm on most mirrorless cameras giving them significantly better AF tracking performance compared to contrast detection.
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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.
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1069:, 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
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The double effort is typically used to mutually compensate for the intrinsical weaknesses of the various methods in order to increase the overall reliability and accuracy or to speed up AF function.
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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
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range, stopping precisely at the point where maximal contrast is detected. This means that lenses designed for phase detection often perform poorly on camera bodies that use contrast detection.
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the camera uses flash focus assist and is set to an operation mode that overrides the flash, it may also disable the focus assist. Thus, autofocus may fail to acquire the subject.
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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
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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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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
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246:, Leica had presented a camera based on their previous development, named Correfot, and in 1978 they displayed an SLR camera with fully operational autofocus.
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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
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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.
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will illuminate the scene and project contrast patterns onto even surfaces, so that phase-detection auto-focus can work under these conditions as well.
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required when there is not enough light to take passive measurements.) Passive autofocusing can be achieved by phase detection or contrast measurement.
448:, 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
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Active AF systems measure distance to the subject independently of the optical system, and subsequently adjust the optical system for correct focus.
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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 (
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models, do not include an autofocus motor and therefore cannot autofocus with lenses that lack an inbuilt motor. Some lenses, such as
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was the first to introduce focusing distance measurement for SLR cameras with the FA and FA* series lenses from 1991. Their first
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Most multi-sensor AF cameras allow manual selection of the active sensor, and many offer automatic selection of the sensor using
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An exception to the two-step approach is the mechanical autofocus provided in some enlargers, which adjust the lens directly.
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605:) also have a few "high-precision" focus points with an additional set of prisms and sensors; they are only active with "
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The speed of the AF system is highly dependent on the widest aperture offered by the lens at the current focal length.
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to determine correct focus. Some AF systems rely on a single sensor, while others use an array of sensors. Most modern
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Fricke, Dierk; Denker, Evgeniia; Heratizadeh, Annice; Werfel, Thomas; Wollweber, Merve; Roth, Bernhard (28 May 2019).
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The assist light (also known as AF illuminator) "activates" passive autofocus systems in low-light and low-
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2.8 and faster). Extended precision comes from the wider effective measurement base of the "range finder".
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Phase detection (PD) is achieved by dividing the incoming light into pairs of images and comparing them.
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need to provide coarse directional information in order to speed up the contrast auto-focusing process.
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were known for successfully applying this system. In the latter case, infrared light is usually used to
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use this method (or a hybrid of both contrast and phase-detection autofocus) when focusing in their
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1238:"Non-Contact Dermatoscope with Ultra-Bright Light Source and Liquid Lens-Based Autofocus Function"
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position. This gives the direction and an estimate of the required amount of focus-ring movement.
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2.8 are generally considered best for focusing speed and accuracy. Faster lenses than this (e.g.:
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secondary image registration (TTL SIR) passive phase detection is often used in film and digital
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Fujifilm
Launches Powerhouse 15X Long Zoom Point and shoot Digital Camera: The FinePix F300EXR
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In a hybrid autofocus system, focus is achieved by combining two or more methods, such as:
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A very common example of combined usage is the phase-detection auto-focus system used in
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Active systems may also fail to focus a subject that is very close to the camera (e.g.,
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system that adjusts the focus of the optical system. A variation of autofocus is an
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light onto the subject, which the camera's autofocus system uses to achieve focus.
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the AF detects this (though it does not change the focus), and a shot is taken.
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modes. A notable exception is Canon digital cameras with Dual Pixel CMOS AF.
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can do it using software on an attached computer, whereas cameras like the
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77 – Pair of reconverging lenses; 80, 81 – Pair of light receiving sections
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Several green focus points/areas indicating where the autofocus has locked
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A rare example of an early hybrid system is the combination of an active
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5.6, whilst professional models can often cope with a widest aperture of
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774: in this section. Unsourced material may be challenged and removed.
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8, which is particularly useful for lenses used in conjunction with
1328:"Fujifilm launches FinePix HS50EXR and HS35EXR high-end superzooms"
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List of Nikon compatible lenses with integrated autofocus-motor
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546:) is widely believed to be a "one measurement, one movement"
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process. PD AF in a focus-locking mode (e.g. "One-Shot" for
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Early passive autofocus system integrated in the lens with
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PD AF in a continuously focusing mode (e.g. "AI Servo" for
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the distance to the subject. Compact cameras including the
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on an automatically or manually selected point or area. An
1295:"Busted! The Myth of Open-loop Phase-detection Autofocus"
1174:, a camera that enables focusing in a postprocessing step
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situations in some cameras. The lamp projects visible or
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the availability of motors in all new developed AF-Lenses
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The data collected from AF sensors is used to control an
1270:"Nikon - Technology - Predictive Focus Tracking System"
1153:, the only polarizer to work with some SLR autofocusers
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One selected green focus point using pinpoint autofocus
1192:"S System: Autofocus – Leica Fotografie International"
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Contrast-detection autofocus is achieved by measuring
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There are various ways to measure distance, including
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of the lens is common in professional AF SLR cameras.
1404:"Articles tagged "learn": Digital Photography Review"
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cameras around the year 2003, including Sony's F707,
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Modern (2014) autofocus single lens reflex camera -
1008:. Trap focus is also possible on some Pentax (e.g.
249:The first mass-produced autofocus camera was the
656:Contrast-detect autofocus is a common method in
1428:showing how contrast detection autofocus works.
1350:, by Kenneth William Caleno, January 28, 2009
8:
1422:showing how phase detection autofocus works.
1384:EOS Documentation Project: Almost Trap Focus
1217:"Milestones - Asahi Optical Historical Club"
163:; Nikon cameras call it "continuous focus".
1041:, some Canon DSLRs can perform trap focus.
378:. Unsourced material may be challenged and
1101:bodies and the more budget-oriented among
308:and switched to the completely electronic
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1053:SLR cameras, and in other brands such as
790:Learn how and when to remove this message
712:flashing is sometimes used to reduce the
398:Learn how and when to remove this message
893:Comparison of active and passive systems
809:Phase detection and contrast measurement
620:Some modern sensors (for example one in
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276:, became the first autofocus 35 mm
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1049:AI Servo is an autofocus mode found on
921:Active autofocus system via infrared -
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674:Mirrorless interchangeable-lens cameras
261:Sonar OneStep was the first autofocus
119:Autofocus systems rely on one or more
329:with AF had been introduced in 1989.
7:
772:adding citations to reliable sources
376:adding citations to reliable sources
1004:to implement trap focusing was the
1437:Canon EOS DSLR Autofocus Explained
1097:Some camera bodies, including all
966:A method variously referred to as
551:are the same closed-loop process.
425:cameras including the Spectra and
30:For the similarly named film, see
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842:aperture-based focus shift errors
131:optical sensors, with a separate
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312:with motorised lenses in 1987.
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759:needs additional citations for
27:Sensor-controlled optical focus
865:EXR. Currently it is used by
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1071:sports and action photography
304:decided to discontinue their
1372:Focus Trap or Catch In Focus
848:Recent developments towards
700:Many cameras and nearly all
257:model released in 1977. The
1432:How Stuff Works – Autofocus
609:" with certain geometrical
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1348:Trap Focus for Nikon Users
826:single-lens reflex cameras
806:Active and passive methods
664:and reflex mirrors. Most
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1125:Counter-examples are the
263:single-lens reflex camera
1389:August 18, 2010, at the
883:Canon EOS 650D/Rebel T4i
651:tracking moving subjects
1426:interactive Flash demo
1420:interactive Flash demo
1075:artificial intelligence
238:Between 1960 and 1973,
1131:Samsung Galaxy S4 Zoom
645:within a sensor field
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173:electronic rangefinder
106:electronic rangefinder
1452:Photography equipment
1167:Manual focus override
1135:Samsung Galaxy K Zoom
1086:types, but are often
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265:, released in 1978.
1362:, Ken Rockwell, 2006
980:wildlife photography
768:improve this article
494:. The system uses a
372:improve this section
147:(DOF) at the widest
532:closed-loop control
1360:How to shoot sport
1313:2010-07-27 at the
1255:10.3390/app9112177
1172:Light-field camera
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756:
754:
747:
741:
738:
726:Sony CyberShot
719:Some external
714:red-eye effect
686:
683:
638:
635:
595:Sony DSLR-A700
569:Some cameras (
474:
471:
465:
462:
450:time-of-flight
406:
405:
356:
354:
347:
341:
338:
327:Pentax lenses
321:
259:Polaroid SX-70
244:photokina 1976
235:
232:
228:teleconverters
145:depth of field
94:control system
88:system uses a
68:
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47:
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1287:
1276:on 2013-11-12
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1198:on 2009-06-21
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1039:Magic Lantern
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1024:cameras. The
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887:Samsung NX300
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757:This section
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711:
706:
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702:camera phones
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584:
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561:
557:
552:
549:
545:
541:
538:, "AF-S" for
537:
533:
529:
525:
521:
518:, "AF-C" for
517:
512:
510:
506:
501:
497:
496:beam splitter
493:
489:
479:
472:
470:
463:
461:
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451:
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443:
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432:
428:
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411:
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367:
366:
362:
357:This section
355:
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345:
339:
337:
335:
330:
328:
325:
317:
313:
311:
307:
303:
299:
297:
292:
290:
287:released the
286:
281:
279:
275:
271:
266:
264:
260:
256:
252:
251:Konica C35 AF
247:
245:
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182:
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111:
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99:
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87:
83:
79:
65:
51:
35:
34:
19:
1398:
1379:
1367:
1355:
1343:
1331:. Retrieved
1322:
1303:
1289:
1278:. Retrieved
1274:the original
1264:
1248:(11): 2177.
1245:
1241:
1231:
1220:. Retrieved
1211:
1200:. Retrieved
1196:the original
1186:
1121:
1096:
1084:
1081:Focus motors
1066:
1048:
999:
991:
987:
983:
975:
971:
967:
965:
907:
900:
896:
855:
847:
834:
823:
815:
812:
801:
786:
780:January 2018
777:
766:Please help
761:verification
758:
724:was used in
718:
710:stroboscopic
707:
699:
688:
679:
655:
640:
619:
575:Canon EOS-1V
568:
560:Canon EOS-1D
556:Canon EOS-1V
553:
513:
485:
467:
459:
454:
412:
409:
394:
385:
370:Please help
358:
331:
314:
300:
296:Minolta 7000
293:
282:
267:
248:
237:
178:
171:
165:
153:
141:
133:sensor array
118:
81:
77:
75:
32:
1107:Pentax' DA*
939:Pentax ME-F
923:Canon AF35M
685:Assist lamp
613:(typically
607:fast lenses
505:front focus
500:rangefinder
492:SLR cameras
442:Canon AF35M
431:triangulate
270:Pentax ME-F
253:, a simple
125:SLR cameras
1280:2013-11-12
1222:2021-08-29
1202:2009-05-15
1179:References
1103:Nikon's DX
1000:The first
972:focus trap
968:trap focus
962:Trap focus
721:flash guns
660:that lack
591:Pentax K-1
509:back focus
444:, and the
415:ultrasonic
388:March 2014
280:in 1981.
183:of around
156:algorithms
33:Auto Focus
18:Auto focus
1099:Canon EOS
1022:Canon EOS
898:systems.
863:Super CCD
670:live-view
611:apertures
603:DSLR-A900
599:DSLR-A850
571:Minolta 7
564:Nikon D2X
359:does not
78:autofocus
42:Autofocus
1446:Category
1387:Archived
1311:Archived
1145:See also
1133:and the
1045:AI Servo
986:but not
955:Nikon D4
869:Series,
858:Fujifilm
838:aperture
736:models.
708:Similar
691:contrast
662:shutters
622:Librem 5
615:f-number
423:Polaroid
419:infrared
310:EF mount
306:FD mount
283:In 1983
161:AI servo
149:aperture
137:metering
1333:June 8,
1030:EOS 40D
530:) is a
464:Passive
380:removed
365:sources
234:History
204:1.4 or
181:F-stops
121:sensors
114:passive
86:optical
1129:, the
1063:Pentax
1061:, and
1026:EOS 1D
1020:, and
984:detect
941:(1981)
925:(1979)
524:Pentax
440:, the
340:Active
324:-mount
316:Pentax
110:active
96:and a
90:sensor
1113:Notes
1055:Nikon
1051:Canon
1018:Nikon
974:, or
875:Ricoh
666:DSLRs
540:Nikon
536:Canon
520:Nikon
516:Canon
435:Nikon
427:SX-70
302:Canon
285:Nikon
240:Leitz
190:2 to
102:focus
98:motor
1335:2013
1059:Sony
1032:and
1012:and
885:and
734:F828
732:and
730:F717
544:Sony
542:and
528:Sony
526:and
363:any
361:cite
294:The
289:F3AF
274:lens
268:The
127:use
92:, a
1250:doi
1090:or
1016:),
1014:K-5
1010:K-x
1002:SLR
988:set
905:).
770:by
587:40D
583:30D
507:or
374:by
278:SLR
230:.
100:to
76:An
1448::
1244:.
1240:.
1094:.
1057:,
1034:7D
970:,
889:.
881:,
877:,
873:,
818:IR
695:IR
601:,
597:,
593:,
589:,
581:,
579:1D
577:,
573:,
562:,
558:,
522:,
336:.
322:AF
112:,
84:)
82:AF
1406:.
1337:.
1297:.
1283:.
1258:.
1252::
1246:9
1225:.
1205:.
1137:.
793:)
787:(
782:)
778:(
764:.
585:/
401:)
395:(
390:)
386:(
382:.
368:.
320:K
224:/
222:f
217:/
215:f
209:/
207:f
202:/
200:f
195:/
193:f
188:/
186:f
80:(
36:.
20:)
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