An autofocus (or AF)
optical
Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultraviole ...
system uses a
sensor
A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon.
In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...
, a
control system
A control system manages, commands, directs, or regulates the behavior of other devices or systems using control loops. It can range from a single home heating controller using a thermostat controlling a domestic boiler to large industrial c ...
and a
motor
An engine or motor is a machine designed to convert one or more forms of energy into mechanical energy.
Available energy sources include potential energy (e.g. energy of the Earth's gravitational field as exploited in hydroelectric power gen ...
to
focus
Focus, or its plural form foci may refer to:
Arts
* Focus or Focus Festival, former name of the Adelaide Fringe arts festival in South Australia Film
*''Focus'', a 1962 TV film starring James Whitmore
* ''Focus'' (2001 film), a 2001 film based ...
on an
automatically or manually selected point or area. An
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
active
Active may refer to:
Music
* ''Active'' (album), a 1992 album by Casiopea
* Active Records, a record label
Ships
* ''Active'' (ship), several commercial ships by that name
* HMS ''Active'', the name of various ships of the British Royal ...
,
passive
Passive may refer to:
* Passive voice, a grammatical voice common in many languages, see also Pseudopassive
* Passive language, a language from which an interpreter works
* Passivity (behavior), the condition of submitting to the influence of on ...
or hybrid types.
Autofocus systems rely on one or more
sensors
A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon.
In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...
to determine correct focus. Some AF systems rely on a single sensor, while others use an array of sensors. Most modern
SLR camera
A single-lens reflex camera (SLR) is a camera that typically uses a mirror and prism system (hence "reflex" from the mirror's reflection) that permits the photographer to view through the lens and see exactly what will be captured. With twin le ...
s use
through-the-lens In photography, through-the-lens metering (TTL metering) refers to a feature of cameras whereby the intensity of light reflected from the scene is measured through the lens; as opposed to using a separate metering window or external hand-held light ...
optical sensors, with a separate sensor array providing light
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
The depth of field (DOF) is the distance between the nearest and the furthest objects that are in acceptably sharp focus in an image captured with a camera.
Factors affecting depth of field
For cameras that can only focus on one object dist ...
(DOF) at the widest
aperture
In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane.
An opt ...
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
In mathematics and computer science, an algorithm () is a finite sequence of rigorous instructions, typically used to solve a class of specific problems or to perform a computation. Algorithms are used as specifications for performing c ...
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 data collected from AF sensors is used to control an
electromechanical
In engineering, electromechanics combines processes and procedures drawn from electrical engineering and mechanical engineering. Electromechanics focuses on the interaction of electrical and mechanical systems as a whole and how the two systems ...
system that adjusts the focus of the optical system. A variation of autofocus is an ''
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.
F-stop
In optics, the f-number of an optical system such as a camera lens is the ratio of the system's focal length to the diameter of the entrance pupil ("clear aperture").Smith, Warren ''Modern Optical Engineering'', 4th Ed., 2007 McGraw-Hill ...
s of around 2 to 2.8 are generally considered best for focusing speed and accuracy. Faster lenses than this (e.g.: 1.4 or 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 5.6, whilst professional models can often cope with a widest aperture of 8, which is particularly useful for lenses used in conjunction with
teleconverter
A teleconverter (sometimes called tele extender) is a secondary lens mounted between a camera and a photographic lens which enlarges the central part of an image obtained by the lens. For example, a 2× teleconverter for a 35 mm camera enl ...
s.
History
Between 1960 and 1973,
Leitz Leitz may refer to several German companies:
*Esselte Leitz GmbH & Co KG, founded by Louis Leitz in 1896, a German manufacturer of office products
**Louis Leitz (1846–1918), German inventor and founder of Esselte Leitz GmbH & Co KG
* Leitz GmbH & ...
(Leica) patented an array of autofocus and corresponding sensor technologies. At
photokina 1976, Leica had presented a camera based on their previous development, named Correfot, and in 1978 they displayed an SLR camera with fully operational autofocus.
The first mass-produced autofocus camera was the
Konica C35 AF
The Konica C35 AF was the first mass-produced autofocus camera. it was first released in November 1977.
Features
This was an autofocus version of the Konica C35 Automatic camera. It featured a fixed- aperture Hexanon 38 mm f/2.8 autofocu ...
, a simple
point and shoot model released in 1977. The
Polaroid SX-70
The SX-70 is a folding single lens reflex Land camera which was produced by the Polaroid Corporation from 1972 to 1981.
History
In 1948, Polaroid introduced its first consumer camera. The Land Camera Model 95 was the first camera to use ins ...
Sonar OneStep was the first autofocus
single-lens reflex camera
A single-lens reflex camera (SLR) is a camera that typically uses a mirror and prism system (hence "reflex" from the mirror's reflection) that permits the photographer to view through the lens and see exactly what will be captured. With twin le ...
, released in 1978.
The
Pentax ME-F
is a brand name used primarily by the Japanese multinational imaging and electronics company Ricoh for DSLR cameras, lenses, sport optics (including binoculars and rifle scopes), and CCTV optics. The Pentax brand is also used by Hoya Corporatio ...
, which used focus sensors in the camera body coupled with a motorized
lens
A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...
, became the first autofocus 35 mm
SLR in 1981.
In 1983
Nikon
(, ; ), also known just as Nikon, is a Japanese multinational corporation headquartered in Tokyo, Japan, specializing in optics and imaging products. The companies held by Nikon form the Nikon Group.
Nikon's products include cameras, camera ...
released the
F3AF, their first autofocus camera, which was based on a similar concept to the ME-F.
The
Minolta 7000
The Minolta MAXXUM 7000 (7000 AF in Europe and α-7000 in Japan) 35 mm SLR camera was introduced in February 1985. It was the first camera to feature both integrated autofocus (AF) and motorised film advance, the standard configuration for later ...
, 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.
Canon
Canon or Canons may refer to:
Arts and entertainment
* Canon (fiction), the conceptual material accepted as official in a fictional universe by its fan base
* Literary canon, an accepted body of works considered as high culture
** Western ca ...
decided to discontinue their
FD mount and switched to the completely electronic
EF mount with motorised lenses in 1987.
Pentax
is a brand name used primarily by the Japanese multinational imaging and electronics company Ricoh for DSLR cameras, lenses, sport optics (including binoculars and rifle scopes), and CCTV optics. The Pentax brand is also used by Hoya Corporation ...
was the first to introduce focusing distance measurement for SLR cameras with the FA and FA* series lenses from 1991. Their first
KAF-mount Pentax lenses with AF had been introduced in 1989.
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 AF-Lenses.
Active
Active AF systems measure distance to the subject independently of the optical system, and subsequently adjust the optical system for correct focus.
There are various ways to measure distance, including
ultrasonic
Ultrasound is sound waves with frequencies higher than the upper audible limit of human hearing. Ultrasound is not different from "normal" (audible) sound in its physical properties, except that humans cannot hear it. This limit varies fr ...
sound waves and
infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
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.
Polaroid
Polaroid may refer to:
* Polaroid Corporation, an American company known for its instant film and cameras
* Polaroid camera, a brand of instant camera formerly produced by Polaroid Corporation
* Polaroid film, instant film, and photographs
* Polar ...
cameras including the Spectra and
SX-70 were known for successfully applying this system. In the latter case, infrared light is usually used to
triangulate
In trigonometry and geometry, triangulation is the process of determining the location of a point by forming triangles to the point from known points.
Applications
In surveying
Specifically in surveying, triangulation involves only angle me ...
the distance to the subject. Compact cameras including the
Nikon
(, ; ), also known just as Nikon, is a Japanese multinational corporation headquartered in Tokyo, Japan, specializing in optics and imaging products. The companies held by Nikon form the Nikon Group.
Nikon's products include cameras, camera ...
35TiQD and 28TiQD, the
Canon AF35M
The Canon AF35M, by Canon Japan or the Sure Shot by Canon USA, was Canon Inc.'s first autofocus 135 film, 35mm lens-shutter Point-and-shoot camera, compact camera. It was launched in November 1979 and received the Japanese Ministry of Interna ...
, and the
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 (ToF) is the measurement of the time taken by an object, particle or wave (be it acoustic, electromagnetic, etc.) to travel a distance through a medium. This information can then be used to measure velocity or path length, or as a w ...
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 devices.
An exception to the two-step approach is the mechanical autofocus provided in some enlargers, which adjust the lens directly.
Passive
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 required when there is not enough light to take passive measurements.) Passive autofocusing can be achieved by phase detection or contrast measurement.
Phase detection
![Autofocus phase detection](https://upload.wikimedia.org/wikipedia/commons/5/5a/Autofocus_phase_detection.svg)
Phase detection (PD) is achieved by dividing the incoming light into pairs of images and comparing them.
Through-the-lens In photography, through-the-lens metering (TTL metering) refers to a feature of cameras whereby the intensity of light reflected from the scene is measured through the lens; as opposed to using a separate metering window or external hand-held light ...
secondary image registration (TTL SIR) passive phase detection is often used in film and digital
SLR cameras SLR may refer to:
Science and technology
* Satellite laser ranging
* Scalable Linear Recording tape drive backup
* Sea level rise
* Self-loading rifle or semi-automatic rifle
** The UK L1A1 SLR rifle
* Semi-linear resolution, a search algorith ...
. The system uses a
beam splitter
A beam splitter or ''beamsplitter'' is an optical device that splits a beam of light into a transmitted and a reflected beam. It is a crucial part of many optical experimental and measurement systems, such as interferometers, also finding wide ...
(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
A rangefinder (also rangefinding telemeter, depending on the context) is a device used to measure distances to remote objects. Originally optical devices used in surveying, they soon found applications in other fields, such as photography an ...
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
Front may refer to:
Arts, entertainment, and media Films
* ''The Front'' (1943 film), a 1943 Soviet drama film
* ''The Front'', 1976 film
Music
*The Front (band), an American rock band signed to Columbia Records and active in the 1980s and ea ...
or
back focus position. This gives the direction and an estimate of the required amount of focus-ring movement.
PD AF in a continuously focusing mode (e.g. "AI Servo" for
Canon
Canon or Canons may refer to:
Arts and entertainment
* Canon (fiction), the conceptual material accepted as official in a fictional universe by its fan base
* Literary canon, an accepted body of works considered as high culture
** Western ca ...
, "AF-C" for
Nikon
(, ; ), also known just as Nikon, is a Japanese multinational corporation headquartered in Tokyo, Japan, specializing in optics and imaging products. The companies held by Nikon form the Nikon Group.
Nikon's products include cameras, camera ...
,
Pentax
is a brand name used primarily by the Japanese multinational imaging and electronics company Ricoh for DSLR cameras, lenses, sport optics (including binoculars and rifle scopes), and CCTV optics. The Pentax brand is also used by Hoya Corporation ...
and
Sony
, commonly stylized as SONY, is a Japanese multinational conglomerate corporation headquartered in Minato, Tokyo, Japan. As a major technology company, it operates as one of the world's largest manufacturers of consumer and professional ...
) is a
closed-loop control
Control theory is a field of mathematics that deals with the control of dynamical systems in engineered processes and machines. The objective is to develop a model or algorithm governing the application of system inputs to drive the system to a ...
process. PD AF in a focus-locking mode (e.g. "One-Shot" for
Canon
Canon or Canons may refer to:
Arts and entertainment
* Canon (fiction), the conceptual material accepted as official in a fictional universe by its fan base
* Literary canon, an accepted body of works considered as high culture
** Western ca ...
, "AF-S" for
Nikon
(, ; ), also known just as Nikon, is a Japanese multinational corporation headquartered in Tokyo, Japan, specializing in optics and imaging products. The companies held by Nikon form the Nikon Group.
Nikon's products include cameras, camera ...
and
Sony
, commonly stylized as SONY, is a Japanese multinational conglomerate corporation headquartered in Minato, Tokyo, Japan. As a major technology company, it operates as one of the world's largest manufacturers of consumer and professional ...
) is widely believed to be a "one measurement, one movement"
open-loop control
In control theory, an open-loop controller, also called a non-feedback controller, is a control system in which the control action is independent of the "process output", which is the process variable that is being controlled."Feedback and contr ...
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.
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
The Nikon D2X is a 12.4-megapixel professional digital single-lens reflex camera (DSLR) that Nikon Corporation announced on September 16, 2004. The D2X was the high-resolution flagship in Nikon's DSLR line until June 2006 when it was supplanted by ...
) 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.
Some cameras (
Minolta 7
The Minolta A-mount camera system was a line of photographic equipment from Minolta introduced in 1985 with the world's first integrated autofocus system in the camera body with interchangeable lenses. The system used a lens mount called A-moun ...
,
Canon EOS-1V,
1D,
30D/
40D,
Pentax K-1
The Pentax K-1 is the first production Pentax full-frame digital SLR camera.
As the flagship model of the Pentax K-mount system, it includes several new and improved features, including a five-axis SR II in-body image stabilization
Image ...
,
Sony DSLR-A700
, commonly stylized as SONY, is a Japanese multinational conglomerate corporation headquartered in Minato, Tokyo, Japan. As a major technology company, it operates as one of the world's largest manufacturers of consumer and professional ...
,
DSLR-A850
The α850 (DSLR-A850) was Sony's second full-frame digital SLR, introduced on 27 August 2009. Similar to the DSLR-A900, the camera featured the same 24.6-megapixel full-frame CMOS sensor used in the a900. On 8 June 2011, Sony pre-announced th ...
,
DSLR-A900) also have a few "high-precision" focus points with an additional set of prisms and sensors; they are only active with "
fast lenses" with certain geometrical
apertures
In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane.
An opt ...
(typically
f-number
In optics, the f-number of an optical system such as a camera lens is the ratio of the system's focal length to the diameter of the entrance pupil ("clear aperture").Smith, Warren ''Modern Optical Engineering'', 4th Ed., 2007 McGraw-Hill ...
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
The Librem 5 is a smartphone manufactured by Purism that is part of their Librem line of products. The phone is designed with the goal of using free software whenever possible, includes PureOS, a Linux operating system, by default, and as of 2021 ...
) include about 2% phase detection pixels on the chip. With suitable software support, that enables phase detection auto focus with suitable software.
Contrast detection
Contrast-detection autofocus is achieved by measuring
Contrast (vision)
Contrast is the contradiction in luminance or colour that makes an object (or its representation in an image or display) distinguishable. In visual perception of the real world, contrast is determined by the difference in the colour and bright ...
within a sensor field
through the lens In photography, through-the-lens metering (TTL metering) refers to a feature of cameras whereby the intensity of light reflected from the scene is measured through the lens; as opposed to using a separate metering window or external hand-held light ...
. 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
tracking moving subjects, since a loss of contrast gives no indication of the direction of motion towards or away from the camera.
Contrast-detect autofocus is a common method in
digital camera
A digital camera is a camera that captures photographs in digital memory. Most cameras produced today are digital, largely replacing those that capture images on photographic film. Digital cameras are now widely incorporated into mobile device ...
s that lack
shutters and reflex mirrors. Most
DSLR
A digital single-lens reflex camera (digital SLR or DSLR) is a digital camera that combines the optics and the mechanisms of a single-lens reflex camera with a digital imaging sensor.
The reflex design scheme is the primary difference between ...
s use this method (or a hybrid of both contrast and phase-detection autofocus) when focusing in their
live-view modes. A notable exception is Canon digital cameras with Dual Pixel CMOS AF.
Mirrorless interchangeable-lens camera
A mirrorless camera is a photo camera featuring a single, removable lens and a digital display.
The camera does not have a reflex mirror or optical viewfinder like a digital single-lens reflex (DSLR) camera, but may have an electronic v ...
s 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.
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 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.
Assist lamp
The assist light (also known as AF illuminator) "activates" passive autofocus systems in low-light and low-
contrast situations in some cameras. The lamp projects visible or
IR light onto the subject, which the camera's autofocus system uses to achieve focus.
Many cameras and nearly all
camera phone
A camera phone is a mobile phone which is able to capture photographs and often record video using one or more built-in digital cameras. It can also send the resulting image wirelessly and conveniently. The first commercial phone with color cam ...
s 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 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.
Similar
stroboscopic flashing is sometimes used to reduce the
red-eye effect
The red-eye effect in photography is the common appearance of red pupils in color photographs of the eyes of humans and several other animals. It occurs when using a photographic flash that is very close to the camera lens (as with most compact ...
, but this is only intended to constrict the subject's eye pupils before the shot.
Some external
flash gun
Flash, flashes, or FLASH may refer to:
Arts, entertainment, and media
Fictional aliases
* Flash (DC Comics character), several DC Comics superheroes with super speed:
** Flash (Barry Allen)
** Flash (Jay Garrick)
** Wally West, the first Kid ...
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
Sony CyberShot cameras around the year 2003, including Sony's F707,
F717 and
F828 models.
Hybrid autofocus
In a hybrid autofocus system, focus is achieved by combining two or more methods, such as:
* Active and passive methods
* Phase detection and contrast measurement
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.
A rare example of an early hybrid system is the combination of an active
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 cameras
A single-lens reflex camera (SLR) is a camera that typically uses a mirror and prism system (hence "reflex" from the mirror's reflection) that permits the photographer to view through the lens and see exactly what will be captured. With twin l ...
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
An autofocus (or AF) optical system uses a sensor, a control system and a motor to focus on an automatically or manually selected point or area. An electronic rangefinder has a display instead of the motor; the adjustment of the optical system ...
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
In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane.
An opt ...
(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 inherit 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
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 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
, trading as Fujifilm, or simply Fuji, is a Japanese multinational conglomerate headquartered in Tokyo, Japan, operating in the realms of photography, optics, office and medical electronics, biotechnology, and chemicals.
The offerings from th ...
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
Super CCD is a proprietary charge-coupled device that has been developed by Fujifilm since 1999. The Super CCD uses octagonal, rather than rectangular, pixels. This allows a higher horizontal and vertical resolution (at the expense of diagonal res ...
EXR. Currently it is used by
Fujifilm FinePix
The Fujifilm FinePix products are a line of digital cameras produced by Fujifilm. They include compact point and shoot models, tough, waterproof models, bridge digital cameras, digital SLRs and mirrorless cameras. Many use Fujifilm's proprietary ...
Series,
Fujifilm X100S,
Ricoh
is a Japanese multinational imaging and electronics company (law), company. It was founded by the now-defunct commercial division of the Riken, Institute of Physical and Chemical Research (Riken) known as the ''Riken concern (business), Concer ...
,
Nikon 1 series
The Nikon 1 series is a discontinued camera line from Nikon, originally announced on 21 September 2011. The cameras utilized Nikon 1-mount lenses, and featured 1" CX format sensors.
The series included the Nikon 1 V1, J1, J2, and S1 with a ...
,
Canon EOS 650D/Rebel T4i and
Samsung NX300
The Samsung NX300 is a mirrorless interchangeable lens digital camera a part of Samsung NX series and a successor to Samsung NX210. It was announced in Jan 2013 along with 45mm f/1.8 2D/3D lens and is the first single-lens system capable of cap ...
.
Comparison of active and passive systems
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 systems.
Active systems may also fail to focus a subject that is very close to the camera (e.g.,
macro photography
Macro photography (or photomacrography or macrography, and sometimes macrophotography) is extreme close-up photography, usually of very small subjects and living organisms like insects, in which the size of the subject in the photograph is grea ...
).
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 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.
File:My Canon AF35M (4307694589).jpg, Active autofocus system via infrared - Canon AF35M
The Canon AF35M, by Canon Japan or the Sure Shot by Canon USA, was Canon Inc.'s first autofocus 135 film, 35mm lens-shutter Point-and-shoot camera, compact camera. It was launched in November 1979 and received the Japanese Ministry of Interna ...
(1979)
File:Pentax ME-F autofocus.jpg, Early passive autofocus system integrated in the lens with Pentax ME-F
is a brand name used primarily by the Japanese multinational imaging and electronics company Ricoh for DSLR cameras, lenses, sport optics (including binoculars and rifle scopes), and CCTV optics. The Pentax brand is also used by Hoya Corporatio ...
(1981)
File:D4-85 1.4.JPG, Modern (2014) autofocus single lens reflex camera - Nikon D4
The Nikon D4 is a 16.2-megapixel professional-grade full frame (35mm) digital single-lens reflex camera (DSLR) announced by Nikon Corporation on 6 January 2012. It succeeds the Nikon D3S and introduces a number of improvements including a 16.2 ...
Trap focus
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
Wildlife photography is a genre of photography concerned with documenting various forms of wildlife in their natural habitat.
As well as requiring photography skills, wildlife photographers may need field craft skills. For example, some anima ...
, 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 – using manual focus to set focus (or switching to manual after focus has been set) but then using ''
focus priority
Most digital cameras support the ability to choose among a number of configurations, or ''mode dial, modes'', for use in various situations. Professional DSLR cameras provide several manual modes; consumer point-and-shoot cameras emphasize autom ...
'' 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, the AF detects this (though it does not change the focus), and a shot is taken.
The first
SLR to implement trap focusing was the
Yashica 230 AF
Yashica was a Japanese manufacturer of cameras, originally active from 1949 until 2005 when its then-owner, Kyocera, ceased production.
In 2008, the Yashica name reappeared on cameras produced by the Hong Kong-based MF Jebsen Group. In 2015, tr ...
. Trap focus is also possible on some Pentax (e.g.
K-x and
K-5),
Nikon
(, ; ), also known just as Nikon, is a Japanese multinational corporation headquartered in Tokyo, Japan, specializing in optics and imaging products. The companies held by Nikon form the Nikon Group.
Nikon's products include cameras, camera ...
, and
Canon EOS
Canon EOS (Electro-Optical System) is an autofocus single-lens reflex camera (SLR) and mirrorless camera series produced by Canon Inc. Introduced in 1987 with the Canon EOS 650, all EOS cameras used 135 film, 35 mm photographic film, film until ...
cameras. The
EOS 1D can do it using software on an attached computer, whereas cameras like the
EOS 40D and
7D 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
Magic Lantern
The magic lantern, also known by its Latin name , is an early type of image projector that used pictures—paintings, prints, or photographs—on transparent plates (usually made of glass), one or more lenses, and a light source. Because a si ...
, some Canon DSLRs can perform trap focus.
AI Servo
AI Servo is an autofocus mode found on
Canon
Canon or Canons may refer to:
Arts and entertainment
* Canon (fiction), the conceptual material accepted as official in a fictional universe by its fan base
* Literary canon, an accepted body of works considered as high culture
** Western ca ...
SLR cameras, and in other brands such as
Nikon
(, ; ), also known just as Nikon, is a Japanese multinational corporation headquartered in Tokyo, Japan, specializing in optics and imaging products. The companies held by Nikon form the Nikon Group.
Nikon's products include cameras, camera ...
,
Sony
, commonly stylized as SONY, is a Japanese multinational conglomerate corporation headquartered in Minato, Tokyo, Japan. As a major technology company, it operates as one of the world's largest manufacturers of consumer and professional ...
, and
Pentax
is a brand name used primarily by the Japanese multinational imaging and electronics company Ricoh for DSLR cameras, lenses, sport optics (including binoculars and rifle scopes), and CCTV optics. The Pentax brand is also used by Hoya Corporation ...
, under the name "continuous focus" (AF-C).
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 and action photography. AI refers to
artificial intelligence
Artificial intelligence (AI) is intelligence—perceiving, synthesizing, and inferring information—demonstrated by machines, as opposed to intelligence displayed by animals and humans. Example tasks in which this is done include speech re ...
: algorithms that constantly predict where a subject is about to be based on its speed and acceleration data from the autofocus sensor.
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 An ultrasonic motor is a type of piezoelectric motor powered by the ultrasonic vibration of a component, the stator, placed against another component, the rotor or slider depending on the scheme of operation (rotation or linear translation). Ultraso ...
s or
stepper motor
A stepper motor, also known as step motor or stepping motor, is a brushless DC electric motor that divides a full rotation into a number of equal steps. The motor's position can be commanded to move and hold at one of these steps without any pos ...
s.
Some camera bodies, including all
Canon EOS
Canon EOS (Electro-Optical System) is an autofocus single-lens reflex camera (SLR) and mirrorless camera series produced by Canon Inc. Introduced in 1987 with the Canon EOS 650, all EOS cameras used 135 film, 35 mm photographic film, film until ...
bodies and the more budget-oriented among
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* 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.
Notes
See also
*
Circular polarizer
A polarizer or polariser is an optical filter that lets light waves of a specific polarization pass through while blocking light waves of other polarizations. It can filter a beam of light of undefined or mixed polarization into a beam of wel ...
, the only polarizer to work with some SLR autofocusers
*
Fixed-focus lens
A photographic lens for which the focus is not adjustable is called a fixed-focus lens or sometimes focus-free. The focus is set at the time of lens design, and remains fixed. It is usually set to the hyperfocal distance, so that the depth of fie ...
*
*
Manual focus override
*
Light-field camera
A light field camera, also known as a plenoptic camera, is a camera that captures information about the ''light field'' emanating from a scene; that is, the intensity of light in a scene, and also the precise direction that the light rays are tr ...
, a camera that enables focusing in a postprocessing step
References
{{Reflist, 30em
External links
interactive Flash demo showing how phase detection autofocus works.
showing how contrast detection autofocus works.
Canon EOS DSLR Autofocus Explained
Photography equipment