What is the difference between AI Focus and AI Servo autofocus modes?

Question

Recently I bought a Canon 60D and I am uncertain about when and how to use/activate the AI Focus and AI Servo functions.

I understand the One Shot autofocus function for still subjects. You press the shutter and the camera/lens will autofocus for you.

With AI Servo AF (for moving subjects), I also press down the shutter button and follow/point towards the subject as it moves around? Till the camera finally finds the autofocus point?

Can someone please explain if this is the correct method?

AI Servo function... I have read the menu, but I am still unsure what this does and when should I use this function.

Lastly, what is the difference of these functions when compared to “panning.”

Answer 1

AI focus works like single shot but if the subject starts moving as you shoot the focus system will track the subject (so that it doesn't move out of the plane of focus). It's good for still subjects that may move unexpectedly.

AI servo continuously tracks the subject as long as you hold down the focus button and is good for continually moving subjects. For example if the subject is moving toward you the camera constantly measures the distance and adjusts the lens so that when you press the shutter the subject will be in focus (in theory, it can get it wrong).

The focus mode is separate to focus point selection, if you select a specific focus point that is the one the camera will use for all AF modes.

Finally I don't understand the last question, panning in a technique to move the camera to track a subject, not an autofocus mode. You should probably us AI servo when panning as you would expect the distance from the subject to camera to change as you pan.