For the film of the same name see Auto Focus. Auto Focus is a 2002 American Biographical film directed by Paul Schrader.

Autofocus (or AF) is a feature of some optical systems that allows them to obtain (and in some systems to also continuously maintain) correct focus on a subject, instead of requiring the operator to adjust focus manually. FOCUS is a software product of Information Builders Inc Originally developed for data handling and analysis on the IBM mainframe, as newer systems were developed

General

Auto-focus systems can capture a subject a variety of ways; here, the focus is on the person's image in the mirror.

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 optical AF sensors, which also perform as light meters. The single-lens reflex (SLR Camera uses an automatic moving mirror system which permits the photographer to see exactly what will be captured by the film or digital imaging Through-the-lens ( TTL) metering is a photographic term describing a feature of Cameras capable of measuring light levels in a scene

The speed and accuracy of through-the-lens optical autofocusing is now often more precise than what can be achieved manually with an ordinary viewfinder. (More precise manual focus can, of course, be achieved with special accessories such as focusing magnifiers. ) Autofocus accuracy within 1/3 of the depth of field (DOF) at the widest aperture of the lens is not uncommon in professional AF SLR cameras. In Optics, particularly as it relates to Film and Photography, the depth of field (DOF is the portion of a scene that appears sharp in the image

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. In Mathematics, Computing, Linguistics and related subjects an algorithm is a sequence of finite instructions often used for Calculation Some AF cameras are able to detect if 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.

The data collected from AF sensors is used to control an electromechanical system that adjusts the focus of the optical system. In Engineering, electromechanics combines the Sciences of Electromagnetism of Electrical engineering and mechanics. A variation of autofocus is called an electronic rangefinder, a system in which focus data are provided to the operator, but adjustment of the optical system is still performed manually. A rangefinder is a device that measures distance from the observer to a target for the purposes of Surveying, determining focus in Photography, or

History

The first mass-produced autofocus camera was the Konica C35 AF, a simple point and shoot model released in 1977. The Konica C35 AF was the first mass-produced Autofocus camera, released in November 1977. A point-and-shoot camera, also called a compact camera, is a Still camera designed primarily for simple operation Also 1977 (album by Ash. Year 1977 ( MCMLXXVII) was a Common year starting on Saturday (link displays The Polaroid SX-70 was the first autofocus single-lens reflex camera, released in 1978. For the Polaroid instant camera see Instant camera. For the company see Polaroid Corporation. The SX-70 is a folding Single lens reflex Land Camera which was produced by the Polaroid Corporation from 1972-1977 The single-lens reflex (SLR Camera uses an automatic moving mirror system which permits the photographer to see exactly what will be captured by the film or digital imaging Year 1978 ( MCMLXXVIII) was a Common year starting on Sunday (link displays the 1978 Gregorian calendar) The Pentax ME-F, which used focus sensors in the camera body coupled with a motorized lens, became the first autofocus 35mm SLR in 1981. The Pentax ME F was an amateur level interchangeable lens 35 mm film, single-lens reflex (SLR Camera. A photographic lens (also known as objective lens or photographic objective) is an optical lens or assembly of lenses used in conjunction with The single-lens reflex (SLR Camera uses an automatic moving mirror system which permits the photographer to see exactly what will be captured by the film or digital imaging Year 1981 ( MCMLXXXI) was a Common year starting on Thursday (link displays the 1981 In 1983 Nikon released the F3AF, their first autofocus camera, which was based on a similar concept to the ME-F. Year 1983 ( MCMLXXXIII) was a Common year starting on Saturday (link displays the 1983 Gregorian calendar) ( also known as Nikon or Nikon Corp, is a Multinational corporation headquartered in Tokyo Japan specializing in Optics The Minolta Maxxum 7000, 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, as it would for Nikon. The Minolta MAXXUM 7000 (7000 AF in Europe 35mm SLR camera was introduced in 1985 Year 1985 ( MCMLXXXV) was a Common year starting on Tuesday (link displays 1985 Gregorian calendar) Canon, however, elected to develop their EOS system with motorised lenses instead. is a Japanese Multinational corporation that specializes in imaging and optical products including Cameras photocopiers and Computer printers

Types

Active autofocus

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 sound waves and infrared light. Infrared ( IR) radiation is Electromagnetic radiation whose Wavelength is longer than that of Visible light, but shorter than that of 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 cameras including the Spectra and SX-70 were known for successfully applying this system. For the Polaroid instant camera see Instant camera. For the company see Polaroid Corporation. In the latter case, infrared light is usually used to triangulate the distance to the subject. 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 Compact cameras including the Nikon 35TiQD and 28TiQD, the Canon AF35M, and the Contax T2 and T3, as well as early video cameras, used this system. ( also known as Nikon or Nikon Corp, is a Multinational corporation headquartered in Tokyo Japan specializing in Optics is a Japanese Multinational corporation that specializes in imaging and optical products including Cameras photocopiers and Computer printers Contax was a Camera brand noted for its unique and sometimes odd technical innovation and a wide range of Zeiss lenses, noted for their high optical

An exception to the two-step approach is the mechanical autofocus provided in some enlargers, which adjust the lens directly.

Passive autofocus

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. An Autofocus assist beam or AF assist beam is a lamp (usually red which is incorporated into photographic cameras and flash units ) Passive autofocusing can be achieved by phase detection or contrast measurement.

Phase detection system

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) is often used in film and digital SLR cameras. Through-the-lens ( TTL) metering is a photographic term describing a feature of Cameras capable of measuring light levels in a scene The single-lens reflex (SLR Camera uses an automatic moving mirror system which permits the photographer to see exactly what will be captured by the film or digital imaging The system uses a beam splitter (implemented as a small semi-transparent area of the main reflex mirror, coupled with a small secondary mirror) to direct light to an AF sensor at the bottom of the camera. A beam splitter is an optical device that splits a beam of Light in two Two optical prisms capture the light rays coming from the opposite sides of the lens and divert it to the AF sensor, creating a simple range finder with a base identical to the lens' diameter. In Optics, a prism is a transparent optical element with flat polished surfaces that refract Light. A rangefinder is a device that measures distance from the observer to a target for the purposes of Surveying, determining focus in Photography, or The two images are then analysed for similar light intensity patterns (peaks and valleys) and the phase difference 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 sensor is typically a one-dimensional photosensitive strip (only a few pixels high and a few dozen wide), some modern cameras (Canon EOS-1D, Nikon D2X) feature Area SIR sensors that are rectangular so as to provide two-dimensional intensity patterns. The Canon EOS-1D is a 4-megapixel professional Digital single-lens reflex camera launched in November 2001 The Nikon D2X is a 124- Megapixel professional Digital single-lens reflex camera that Nikon Corporation announced on September 16, 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-1D, Canon EOS 30D/40D) ) also have a few 'high precision' focus points with additional set of prisms and sensors; they are only active with 'fast lenses' of certain focal ratio. The Canon EOS 30D is an 82- Megapixel semi-professional Digital single-lens reflex camera, initially announced on February 20, The Canon EOS 40D is a 101- Megapixel semi-professional Digital single-lens reflex camera. Lens speed refers to the maximum Aperture diameter or minimum F-number, of a Photographic lens. Extended precision comes from the increased diameter of such lenses, so the base of the 'range finder' can be wider.

Contrast measurement is achieved by measuring contrast within a sensor field, through the lens. Through-the-lens ( TTL) metering is a photographic term describing a feature of Cameras capable of measuring light levels in a scene 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. This is a common method in video cameras and consumer-level digital cameras that lack shutters and reflex mirrors. A video camera is a Camera used for electronic Motion picture acquisition initially developed by the Television industry but now common in other applications A point-and-shoot camera, also called a compact camera, is a Still camera designed primarily for simple operation 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 Some DSLRs (Olympus E-420, Panasonic L10, Nikon D300 in Tripod Mode) use this method when focusing in their live-view modes. A digital single-lens reflex camera ( digital SLR or DSLR) is a Digital camera that uses an automatic mirror system and Pentaprism The Olympus E-420 (or Olympus EVOLT E-420 in North America) is a 10 megapixel digital single-lens reflex (dSLR camera that conforms to the Four Thirds The Lumix DMC-L10 is Panasonic 's second Digital single-lens reflex camera (DSLR a follow-up to the previous Lumix DMC-L1 model In Digital photography, live preview is the feature that allows a Digital camera 's electronic display to be used as a Viewfinder, that is as a means of

Pros and cons of the 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 is often considerably less than passive systems.

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

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 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.

See also

• Fixed focus
• Circular polarizer, the only polarizer to work with some SLR autofocuses

References

• Norman Goldberg. A fixed-focus lens is a lens of which the focus is not adjustable In Photography, a filter is a Camera accessory consisting of an optical filter that can be inserted in the optical path Camera Technology : The Dark Side of the Lens
• Sidney Ray. Applied Photographic Optics
• Ralph Jacobson, Sidney Ray, Geoffrey G Attridge, Norman Axford. Manual of Photography: Photographic and Digital Imaging
• J. M. Geusebroek, F. Cornelissen, A. W. M. Smeulders, and H. Geerts. Robust autofocusing in microscopy. Cytometry, 39(1):1-9, 2000.

External links

• How Autofocus Cameras Work
• Nikon's Article on the Predictive Focus Tracking System
• Detailed Explanation of Contrast Detection Method
• Principle of the Split Image Focusing Aid and the Phase Comparison Autofocus Detector in Single Lens Reflex Cameras
• AI Servo Information by Canon USA
• Autofocus in SLR Cameras, from Juza Nature Photography