I am interested in how works AI SERVO in terms of metering. As it tracks objects, when is the exposure set and how can I set it manually? Is it set automatically when the shutter button is pressed?
You seem to mix up focus features with metering features. You manual section on AI focus is assuming that you are in auto metering mode. it will keep tracking metering as well as focus in that mode, ASSUMING you have auto metering on. The metering mode, however, is set independently. You have your auto and semi auto modes. full auto, P mode (you chose between permutations of T and A, and then you have the "fix one, auto the other, Av and Tv, and you have full manual. (lets ignore ISO for simplicity). On top of that you have the asterix to lock an auto exposure. Oh, and you also have exposure compensation if you dont like the automatic one given the metering mode you chose "spot", mean, evaluative. Oh crap, there goes the simplicity. But the point is, if you leave any kind of automatic adjustment, the final adjustment is made as you snap the photo. No matter what focus mode you chose. You can even follow the changes it makes to exposure while tracking the focus in the viewfinder. And you can lock it with asterix.
The answer to your question as written is:
- The exposure is set at the point the picture is taken, or put another way, when the shutter button is fully depressed. From previous questions, you've said you're using a Canon 550D; this information is available on page 67 of the manual.
- You can set the exposure manually by use of manual mode on your camera; if all you need is an offset from the camera's exposure, you can use exposure compensation.
None of this is specific to AI Servo focus mode though - it applies in every focus mode, even manual focus. The only exception is if you are using One Shot focus and evaluative metering, when half-pressing the shutter button gives you exposure lock.