for: Auto Focus
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for: Auto Focus
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.
General
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.
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 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. 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.
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 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 teleconverters.
History
The first mass-produced autofocus camera was the Konica C35 AF, a simple point and shoot model released in 1977. The Polaroid SX-70 was the first autofocus single-lens reflex camera, released in 1978. The Pentax ME-F, which used focus sensors in the camera body coupled with a motorized lens, became the first autofocus 35 mm SLR in 1981. In 1983 Nikon released the F3AF, their first autofocus camera, which was based on a similar concept to the ME-F. 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. Canon, however, elected to develop their EOS system with motorised lenses instead.
