Autofocus

An autofocus (or AF) optical system uses a sensor, a control system and a motor to focus 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.

There are 3 method :
Active focus

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. 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. In the latter case, infrared light is usually used to triangulate the distance to the subject. 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.

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

Passive focus

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.

Hybrid focus
In a hybrid autofocus system, focus is achieved by combining two or more methods, such as:

1.     Active and passive methods.
2.     Phase detection and contrast measurement.

The double effort is typically used to speed up and improve AF function. In July, 2010, Fujifilm 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 EXR. Currently it is used by Fujifilm FinePix Series, Ricoh, Nikon 1 series and Canon EOS 650D/Rebel T4i.

source : wikipedia.org