This is an extension of a thread I began and titled XSi and focus issue, but this question is specifically about using back button focus with the XSi and AI Servo. I've read the Canon link on back button focus but it's descriptions of settings cover other, higher end, cameras.
I want to really understand the 4 choices I'm offered in the XSi Custom Function-10 that deals with this. I'm particularly confused by the choice between #2 and #3. The manual says that for choice 2: "AF/AF lock, no AE lock In the AI Servo AF mode, you can press the * button to stop the AF operation momentarily. This prevents the AF from being thrown off by any obstacle passing between the camera and subject. The exposure is set at the moment the moment the picture is taken."
For choice #3 it says: "AE/AF, no AE lock This is useful for subjects which keep moving and stopping repeatedly. In AI Servo AF mode, you can press the * button to start or stop the AI Servo operation. The exposure is set at the moment the picture is taken. Thus the optimum focusing and exposure will always be achieved as you wait for the decisive moment."
If the exposure is achieved at the moment the picture is taken with both settings, what's the difference from an exposure point of view and why would you choose one setting over the other?
What's the difference from a focus point of view?