Autofocus
Encyclopedia
An autofocus optical system uses a sensor
Sensor
A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury-in-glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated...

, a control system
Control system
A control system is a device, or set of devices to manage, command, direct or regulate the behavior of other devices or system.There are two common classes of control systems, with many variations and combinations: logic or sequential controls, and feedback or linear controls...

 and a motor
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...

 to focus
Focus (optics)
In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by...

 fully automatic or on a 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. The methods are named by the used sensor: active, passive and hybrid.

General

Autofocus systems rely on one or more sensors to determine correct focus. Some AF systems rely on a single sensor, while others use an array of sensors. Most modern SLR cameras use through-the-lens
Through-the-lens metering
Through-the-lens metering is a photographic term describing a feature of cameras capable of measuring light levels in a scene through their taking lenses, as opposed to a separate metering window...

 optical AF sensors, with a separate sensor array providing light metering
Metering mode
In photography, the metering mode refers to the way in which a camera determines the exposure.- Examples of metering modes :Cameras generally allow the user to select between spot, center-weighted average, or multi-zone metering modes....

, 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 now often speedier and more precise than can be achieved manually 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
Depth of field
In optics, particularly as it relates to film and photography, depth of field is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image...

 (DOF) at the widest aperture
Aperture
In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are,...

 of the lens is not uncommon 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 – a function used mainly in sports and other action photography; on Canon cameras this is known as AI servo, while on Nikon cameras 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 called an electronic rangefinder
Rangefinder
A rangefinder is a device that measures distance from the observer to a target, for the purposes of surveying, determining focus in photography, or accurately aiming a weapon. Some devices use active methods to measure ; others measure distance using trigonometry...

, 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 maximum aperture offered by the lens. F-stops of around 2 to 2.8 are generally considered optimal in terms of 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 maximum aperture of at least 5.6, while professional models can often cope with lenses that have a maximum aperture of 8, which is particularly useful for lenses used in conjunction with teleconverter
Teleconverter
A teleconverter is a secondary lens which is mounted between the camera and a photographic lens. Its job is to enlarge the central part of an image obtained by the objective lens...

s.

History

Between 1960 and 1973, Leitz (Leica)http://www.lfi-online.com/ceemes/page/show/issue_4_09_3 patented an array of autofocus and corresponding sensor technologies. At photokina
Photokina
The photokina is the world's largest trade fair for the photographic and imaging industries. The first photokina was held in Cologne, Germany, in 1950, and it is now held biannually in September at the koelnmesse Trade Fair and Exhibition Centre...

 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
Konica C35 AF
The Konica C35 AF was the first mass-produced autofocus camera, released in November 1977. It featured a fixed-aperture Hexanon 38 mm f/2.8 lens, a built-in electronic flash, and an automatic exposure system to select the appropriate shutter speed. The C35 employed a leaf shutter with three speeds,...

, a simple point and shoot
Point and shoot camera
A point-and-shoot camera, also called a compact camera, is a still camera designed primarily for simple operation. Most use focus free lenses or autofocus for focusing, automatic systems for setting the exposure options, and have flash units built in....

 model released in 1977. The Polaroid
Polaroid Corporation
Polaroid Corporation is an American-based international consumer electronics and eyewear company, originally founded in 1937 by Edwin H. Land. It is most famous for its instant film cameras, which reached the market in 1948, and continued to be the company's flagship product line until the February...

 SX-70
Polaroid SX-70
The SX-70 is a folding single lens reflex Land Camera which was produced by the Polaroid Corporation from 1972-1981.- History :Though Polaroid had considered a Henry Dreyfus-designed SLR for its Colorpack film, the SX-70 was the first instant SLR and the first camera to use Polaroid's new SX-70...

 Sonar OneStep was the first autofocus single-lens reflex camera
Single-lens reflex camera
A single-lens reflex camera is a camera that typically uses a semi-automatic moving mirror system that permits the photographer to see exactly what will be captured by the film or digital imaging system, as opposed to pre-SLR cameras where the view through the viewfinder could be significantly...

, released in 1978. The Pentax ME-F
Pentax ME F
The Pentax ME F was an amateur level, interchangeable lens, 35 mm film, single-lens reflex camera. It was manufactured by Asahi Optical Co., Ltd. of Japan from November 1981 to 1984...

, which used focus sensors in the camera body coupled with a motorized lens
Photographic lens
A camera lens is an optical lens or assembly of lenses used in conjunction with a camera body and mechanism to make images of objects either on photographic film or on other media capable of storing an image chemically or electronically.While in principle a simple convex lens will suffice, in...

, became the first autofocus 35 mm SLR
Single-lens reflex camera
A single-lens reflex camera is a camera that typically uses a semi-automatic moving mirror system that permits the photographer to see exactly what will be captured by the film or digital imaging system, as opposed to pre-SLR cameras where the view through the viewfinder could be significantly...

 in 1981. In 1983 Nikon
Nikon
, also known as just Nikon, is a multinational corporation headquartered in Tokyo, Japan, specializing in optics and imaging. Its products include cameras, binoculars, microscopes, measurement instruments, and the steppers used in the photolithography steps of semiconductor fabrication, of which...

 released the F3AF
Nikon F3
The Nikon F3 was Nikon's third professional single-lens reflex camera body, preceded by the F and F2. Introduced in 1980, it had manual and semi-automatic exposure control whereby the camera would select the correct shutter speed . The Nikon F3 series cameras had the most model variations of any...

, their first autofocus camera, which was based on a similar concept to the ME-F. The Minolta Maxxum 7000
Minolta Maxxum 7000
The Minolta MAXXUM 7000 35mm SLR camera was introduced in 1985. It marked a significant milestone in photography as it was the first camera to feature both integrated autofocus and motorised film advance, the standard configuration for later amateur and professional single lens reflex...

, released in 1985 and badged Dynax in some markets, 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, as it would for Nikon. Canon, however, elected to develop their EOS system with motorised lenses instead. More recently, Nikon have also adopted this strategy with their AF-S range of lenses; their entry-level DSLRs do not have a focus motor in the body.

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
Ultrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing. Ultrasound is thus not separated from "normal" sound based on differences in physical properties, only the fact that humans cannot hear it. Although this limit varies from person to person, it is...

 sound waves and infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

 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
Instant camera
The instant camera is a type of camera that generates a developed film image. The most popular types to use self-developing film were formerly made by Polaroid Corporation....

 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
Triangulation
In trigonometry and geometry, triangulation is the process of determining the location of a point by measuring angles to it from known points at either end of a fixed baseline, rather than measuring distances to the point directly...

 the distance to the subject. Compact cameras including the Nikon
Nikon
, also known as just Nikon, is a multinational corporation headquartered in Tokyo, Japan, specializing in optics and imaging. Its products include cameras, binoculars, microscopes, measurement instruments, and the steppers used in the photolithography steps of semiconductor fabrication, of which...

 35TiQD and 28TiQD, the Canon
Canon Inc.
is a Japanese multinational corporation that specialises in the manufacture of imaging and optical products, including cameras, camcorders, photocopiers, steppers and computer printers. Its headquarters are located in Ōta, Tokyo, Japan.-Origins:...

 AF35M, and the Contax
Contax
Contax was a camera brand noted for its unique technical innovation and a wide range of Zeiss lenses, noted for their high optical quality. Its final incarnation was a line of 35 mm, medium format and digital cameras engineered and manufactured by Kyocera, and featuring modern Zeiss optics...

 T2 and T3, as well as early video cameras, used this system.

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

Phase detection is achieved by dividing the incoming light into pairs of images and comparing them. SIR TTL passive phase detection (secondary image registration, through the lens
Through-the-lens metering
Through-the-lens metering is a photographic term describing a feature of cameras capable of measuring light levels in a scene through their taking lenses, as opposed to a separate metering window...

) is often used in film and digital SLR cameras
Single-lens reflex camera
A single-lens reflex camera is a camera that typically uses a semi-automatic moving mirror system that permits the photographer to see exactly what will be captured by the film or digital imaging system, as opposed to pre-SLR cameras where the view through the viewfinder could be significantly...

. The system uses a beam splitter
Beam splitter
A beam splitter is an optical device that splits a beam of light in two. It is the crucial part of most interferometers.In its most common form, a rectangle, it is made from two triangular glass prisms which are glued together at their base using Canada balsam...

 (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
Rangefinder
A rangefinder is a device that measures distance from the observer to a target, for the purposes of surveying, determining focus in photography, or accurately aiming a weapon. Some devices use active methods to measure ; others measure distance using trigonometry...

 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 if the object is in front focus or back focus position. This instantly gives the exact direction of focusing and amount of focus ring's movement.
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-1v
The Canon EOS-1v is a 35mm single-lens reflex camera from Canon's EOS series, released in 2000, it is the final film camera in Canon's landmark EOS-1 series of professional cameras...

, Canon EOS-1D
Canon EOS-1D
The Canon EOS-1D is a 4-megapixel professional digital single-lens reflex camera launched in November 2001. It was Canon's first professional-level digital camera developed and released by themselves, the previous D2000 being a collaborative effort with Kodak. It has a 1.3x crop factor with a CCD...

, Nikon D2X
Nikon D2X
The Nikon D2X is a 12.4 megapixel professional digital single-lens reflex camera 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 the D2Xs and later on by two new flagship cameras: the...

) feature Area SIR sensors that are rectangular in shape and provide two-dimensional intensity patterns for a finer-grain analysis. Cross-type (CT) 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 30D
The Canon EOS 30D is an 8.2-megapixel semi-professional digital single-lens reflex camera, initially announced on February 21, 2006. It is the successor of the Canon EOS 20D, and is succeeded by the EOS 40D...

/40D
Canon EOS 40D
The Canon EOS 40D is a 10.1-megapixel semi-professional digital single-lens reflex camera. It was initially announced on August 20, 2007 and was released at the end of that month. It is the successor of the Canon EOS 30D, and is succeeded by the EOS 50D. It can accept EF and EF-S lenses...

) also have a few 'high precision' focus points with an additional set of prisms and sensors; they are only active with 'fast lenses' of certain focal ratio
F-number
In optics, the f-number of an optical system expresses the diameter of the entrance pupil in terms of the focal length of the lens; in simpler terms, the f-number is the focal length divided by the "effective" aperture diameter...

. Extended precision comes from the increased diameter of such lenses, so the base of the 'range finder' can be wider.

Contrast measurement

Contrast measurement is achieved by measuring contrast within a sensor field, through the lens
Through-the-lens metering
Through-the-lens metering is a photographic term describing a feature of cameras capable of measuring light levels in a scene through their taking lenses, as opposed to a separate metering window...

. The intensity difference between adjacent pixels of the sensor naturally increases with correct image focus. The optical system can thereby be adjusted until the maximum 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 is a common method in video cameras and consumer-level digital cameras that lack shutter
Shutter (photography)
In photography, a shutter is a device that allows light to pass for a determined period of time, for the purpose of exposing photographic film or a light-sensitive electronic sensor to light to capture a permanent image of a scene...

s and reflex mirrors. Most DSLR
Digital single-lens reflex camera
Most digital single-lens reflex cameras are digital cameras that use a mechanical mirror system and pentaprism to direct light from the lens to an optical viewfinder on the back of the camera....

s use this method (or a hybrid of both contrast and phase detection autofocus) when focusing in their live-view modes. Mirrorless interchangeable-lens cameras, including Micro Four Thirds, exclusively use contrast measurement autofocus except the new Nikon 1 series
Nikon 1 series
The Nikon 1 series are high-speed mirrorless interchangeable-lens cameras. Announced on 21 September 2011, Nikon claims that it is "Nikon's most significant announcement since we introduced our first digital camera 14 years ago"...

. Their manufacturers claim performance comparable or even faster than phase detection systems , though it is unclear under what conditions (e.g., dim light) these claims hold true.

Technically it can be implemented as high pass filter and some system which conscientiously moves lens around the point where filter output is highest. Digital processing is not required. Photo cameras sweep only once before taking picture, while video cameras need to sweep continuously in small steps that are almost unnoticeable.
This method can also be used to precisely focus security cameras, without seeing actual image.

Assist lamp

The assist lamp "activates" passive autofocus systems in low-light situations in some cameras (Nikon). The lamp projects visible or IR
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

 light onto the subject, which the camera's autofocus system uses to achieve focus. Many cameras that do not have a dedicated autofocus assist lamp instead use their built-in flash, illuminating the subject with stroboscopic bursts of light. The strobe bursts aid the autofocus system in the same fashion as a dedicated assist light, but have the disadvantage of startling or annoying living subjects. 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 autofocus may fail to acquire the subject. Similar stroboscopic flashing is sometime used to reduce the red-eye effect
Red-eye effect
The red-eye effect in photography is the common appearance of red pupils in color photographs of eyes. It occurs when using a photographic flash very close to the camera lens , in ambient low light. The effect appears in the eyes of humans and animals that have no tapetum lucidum, hence no...

, but this method is only intended to constrict the subject's eye pupils prior to the actual shot being taken, and thus reduce retinal reflections.

In some cases, external flash guns have integrated autofocus assist lamps that replace the stroboscopic on-camera flash. 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

Autofocus is achieved by combining minimum 2 methods at one time:
  • Active and passive methods.
  • Phase detection and contrast measurement.


The double effort is typically used to speed-up and improve AF function. In July, 2010, Fujifilm
Fujifilm
is a multinational photography and imaging company headquartered in Tokyo, Japan.Fujifilm's principal activities are the development, production, sale and servicing of color photographic film, digital cameras, photofinishing equipment, color paper, photofinishing chemicals, medical imaging...

 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
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...

 EXR.

Currently it is used by Fujifilm FinePix
Fujifilm FinePix
The Fujifilm FinePix products are a line of digital cameras produced by Fujifilm. They include compact point and shoot models, bridge digital cameras, and digital SLRs...

 Series, Ricoh
Ricoh
or Ricoh, is a Japanese company that was established in 1936 on February 6th, as , a company in the RIKEN zaibatsu. Its headquarters is located in Ricoh Building in Chūō, Tokyo....

 and Nikon 1 series
Nikon 1 series
The Nikon 1 series are high-speed mirrorless interchangeable-lens cameras. Announced on 21 September 2011, Nikon claims that it is "Nikon's most significant announcement since we introduced our first digital camera 14 years ago"...

.

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
Macrophotography is close-up photography, usually of very small subjects. Classically a macrophotograph is one in which the size of the subject on the negative is greater than life size. However in modern use it refers to a finished photograph of a subject at greater than life size...

).

Passive systems may not find focus when the contrast is low, notably on large single-coloured 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.

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, 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 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.

Trap focus is possible on some Pentax, Nikon, and Canon EOS
Canon EOS
The Canon EOS autofocus 35 mm film and digital SLR camera system was introduced in 1987 with the Canon EOS 650 and is still in production as Canon's current DSLR system...

 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.

AI servo

AI servo is an auto focus
Auto Focus
Auto Focus is a 2002 American biographical film directed by Paul Schrader that stars Greg Kinnear and Willem Dafoe. The screenplay by Michael Gerbosi is based on the book The Murder of Bob Crane by Robert Graysmith....

 mode found on Canon
Canon Inc.
is a Japanese multinational corporation that specialises in the manufacture of imaging and optical products, including cameras, camcorders, photocopiers, steppers and computer printers. Its headquarters are located in Ōta, Tokyo, Japan.-Origins:...

 SLR cameras, although the same principle is used with Nikon
Nikon
, also known as just Nikon, is a multinational corporation headquartered in Tokyo, Japan, specializing in optics and imaging. Its products include cameras, binoculars, microscopes, measurement instruments, and the steppers used in the photolithography steps of semiconductor fabrication, of which...

 cameras, known as "Continuous Focus". 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
Artificial intelligence is the intelligence of machines and the branch of computer science that aims to create it. AI textbooks define the field as "the study and design of intelligent agents" where an intelligent agent is a system that perceives its environment and takes actions that maximize its...

: algorithms that constantly predict where a subject is about to be based on its speed and acceleration data from the autofocus sensor.

See also


External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
x
OK