The automatic focusing of a camera lens by electronic means on a chosen part of the subject to ensure a sharp image. Various methods are used; for example, a sound-ranging system measures distance from the return time of the echo of an emitted ultrasonic pulse. Single lens reflex cameras use a phase (separation) detection method. The apex of the cone of light from the camera lens is judged to be in focus, convergent, or divergent by its separation into two beams, using small lenses in front of a linear charge-coupled device array, and the spread (phase) is then measured from the response of the array. The data controls a small motor which drives the lens to correct focus.
For the film of the same name click here, 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
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. (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 is provided to the operator, but adjustment of the optical system is still performed manually. 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 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.
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. 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.
Passive autofocus
Passive AF systems determine correct focus by performing passive analysis of the image that is entering the optical system. (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 is achieved by dividing the incoming light into pairs of images and comparing them. The system utilizes 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. Although AF sensor is typically an 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 in 2D.
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.
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 natural illumination to the subject, while active systems may focus correctly even in total darkness.
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