This setting is actually a combination of the first two. When selected, the camera focuses using One Shot AF and locks in the focus setting. But, if the subject begins moving, it will switch automatically to AI Servo AF and change the focus to keep the subject sharp. AI Focus AF is a good choice when you’re shooting a mixture of action pictures and less dynamic shots and want to use One Shot AF when possible. The camera will default to that mode, yet switch automatically to AI Servo AF when it would be useful for subjects that might begin moving unexpectedly.