AI One Shot, AI Servo, AI Focus

Latest e-mail about photography was about these modes. On DSLR Canon cameras you can find three AF modes: One-Shot AF, AI Servo and AI Focus. What is the difference between them?

Note: I will use term “focus button” – what button you use for focusing depends on your camera settings. You can use shutter button, * (asterix) button or specialized AF button (not available on all cameras). We will talk about using different methods in the future.
Note: AI is abbreviation for Artificial Intelligence.

One-Shot AF is for still subjects. When you press focus button, the camera locked focus at your focus point. Now you can recompose (if you use shutter button you should hold button pressed halfway) and shot. The focus will remain on what you focused.
AI Servo for moving subjects. When you press (and hold!) focus button, camera focus on subject and try to refocus if subject will approaches or retreats. Canon cameras use Predictive AF for tracking. Camera determine the speed and direction of moving subjects, then focuses the camera lens to a predicted position to increase the probability of obtaining a sharp photograph.
Camera uses selected AF point for tracking, so you must be sure, your subject is covered with selected AF point (when you move your camera between shooting moving subject).
If you use automatic AF point selection, camera will start with center AF point and then try to track with all available (depends on camera).
Simple example – child is running to you. Press focus button to focus on child and HOLD pressing while child is running to you. Camera will track child and focus will be always available. When you want, you press shutter button and the picture will be taken. If you use Continues mode (what is reasonably with AI Servo), you can hold shutter button pressed and camera will take photos as quick as it can. And the child will be in focus, because camera is tracking the moving!!!
The best way to learn using this is going to sport event and try, try, try …

AI Focus – mixed mode. You can focus like in One-Shot AF mode, but if subject starts moving, camera will start to use AI Servo. Because it is unpredictable, I don’t recommend using this mode.

Personally I use AI Servo all the time. But I use one special setting (Custom function) so camera let me focus like One-shot if I shoot still subjects with recomposition technique.