Autofocus modes

Autofocus modes – autofocus can either activate until a lock is found (single, AF-S) or be continuously active (continuous, AF-C, servo). Single mode is especially used for stationary subjects, when focus, once found, should stay fixed, while continuous mode is instead used for moving subjects. Some AF systems also include anticipation of position of moving subjects – Canon calls this "AI servo" (for "artificial intelligence") – or can automatically switch between single and continuous depending on whether the subject is moving – Canon calls this "AI focus".


A separate but often related distinction is between focus priority and release priority – whether the camera will take a picture when the subject is out of focus or not. In focus priority, the camera will only take a picture when the subject is in focus (as detected by the AF system), while in release priority, the camera will take a picture whenever the shutter is pressed.Release priority is particularly used of fast-moving subjects, which may not be perfectly in focus, or by experienced photographers, who wish to override the camera's judgment of whether the scene is in focus.

These are usually combined – for stationary subjects, AF is set to single (lock when found) and release is set to focus priority, while for moving subjects, AF is set to continuous and release is set to release priority. Manual focus is generally in release priority – AF is neither detected nor set.

Note that these "priority" modes should not be confused with the same word in exposure modes.

Focus priority can also be used for the trap focus trick, to take a picture only when a subject hits a focus point, by using AF to detect focus but not set it.

Source: http://en.wikipedia.org/wiki/List_of_digital_camera_modes