Originally posted by OregonJim Suppose you've got the camera's AF system tracking a moving target as you describe, and another moving object enters the frame that you now want to be the subject (think horse or car race). How easy/fast is it to get the camera to "un-track" the old subject and focus on the new?
Sounds like the creative decision making has shifted from the photographer to the camera...
If your original subject is still being tracked and you want to switch, you have some options. The easiest way is to just release the focus button (for this type of work most people reassign the focus to the rear * button rather than half shutter press), center on the new subject, and hit the focus button again. It'll pick up and track that subject from now on. So it is about as easy and fast as it gets. (You can do some stuff with selecting particular AF points, AF point expansion and so on, but I never get fancy like that.)
Canon published a pretty good PDF on tips and tricks for making the most out of the AF system, and if you're really interested I can send it to you.
As for "shifting the creative decision", the camera still does what you tell it to do, just like with metering and auto exposure. The key is understanding how the camera works so you can use the nice automation to your advantage. Smart AF systems are no more evil or creativity destroying than motor winders or auto exposure. They are just tools.
PS: Would you still be concerned about this if it was Pentax who had this focusing system?