Auto focus refers to the action the camera takes to FOCUS the lens on a specific subject. There are two types of auto focus, namely: Active- and Passive Auto Focus.
Active Auto Focus: used by less expensive cameras, it means that the camera is sending “something” out and calculating Focus Distance. The camera emits a sound, light or infra red light which it then uses to calculate distance by triangulation, reflected intensity or the time it takes to return. The problems facing this type of autofocus is that other forms of infrared light (fire, candles, etc) interfere with the system. It is also “focused” on the centre of the image frame and struggles to focus if the object is more than 6 meters away from you or if there are strong BACKLIGHTS in the image frame
The advantages is that it can work in total darkness, improving your chances of catching an in-focus image with flash.
Passive Auto Focus: Used on most SLR’s, this type of focusing system actually employs a CCD to measure contrast in the scene. The camera will keep on moving the lens’ focusing elements across a the focus field until it is satisfied that it has reached the point of biggest contrast. The advantage of this system is that it has no problem with limited distances, and is very effective.
It’s biggest disadvantage is that you need ambient light to assist in the focusing operation. Most SLR’s have tried to overcome this problem by combining the two systems, either directly on the camera, or by installing infra-red emitters in the off-camera Flash units.