AI Focus vs AI Servo vs One Shot

This is a note to myself. I learned it the hard way, that means I made mistakes to learn this.

AI Focus is for you to focus on something originally stationary, then it starts moving. If you focus on a static subject, and then you move your camera away, the focus will continue to change. There is no focus lock. This mode works very poorly if you are thinking of focusing on a static subject, lock the focus and then point to something else. In the Canon 20D, the focusing points will continue to light up, signalling to you which area is in focus.

AI Servo is for you to focus on something that is ALREADY moving, and you want to continue to focus on that subject. The camera's focusing points won't light up. You will have no idea which point is in focus which is not. The use of this focusing mode can be quite terrible, and many pictures will turn out out of focus. If you use this mode to shoot static subjects, you might kick yourself for not even being able to focus on some simple, static subjects.

One Shot is for you to focus on static subject.

All the above are actually written in the camera manual, just that I don't really understand everything after reading the manual. Nothing is better than practice to learn something, although it may be the hard way.

As I used the wrong mode in a recent shoot, many photos are out of focus, even some simple ones. Sad, sad sad...