What is the difference between AI Focus and AI Servo autofocus modes?
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.”
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.
Not sure about Canon modes, but on a Nikon the continuous focus mode allows the shutter to fire even if the shot is not in focus. (At least by default.) The difference is huge: I tried to get a picture of parrots flying overhead in single-focus mode and ended up with nothing, since exact focus never locked. I switched to continuous focus and even though several of the shots are not in perfect focus, I was at least able to get the shots off and get *something*.
On at least some Canon bodies (like the 7d) the AF system is configurable and you can set whether you want focus lock or shutter priority on taking the image.