Introduction to the Canon 5D Mark II ; vol. 1: Basic Controls Training DVD by Blue Crane Digital

we've already talked a little bit about selecting a focus point and where to focus the camera but how do you decide when to focus you may be taking a portrait of someone standing completely still or you may be at the zoo where an animal's movement is unpredictable when the camera focuses will have an impact on the quality of your image the 5d has a setting appropriate to each of these situations and one for everything in between you can access the autofocus mode options on the quick control screen or press AF Drive and turn the main dial to choose an AF mode your options are one shot ai focus and AI servo one shot freezes the focus when you press the shutter release halfway if a proper focus is obtained the focus point that achieved focus will light up in red the focus indicator will appear and the camera will sound a soft beep this setting is good for stationary shots such as portraits