Welcome Warren. Yes AI servo keeps changing — more specifically, actively tracking your subject — during a burst or even when you're not shooting at all as long as you keep AF active, which you do with a half shutter press under CF4-0 or -2, or by holding down * with CF4-1 or -3.
This means when you're shooting action with AI servo, you have to keep your focus point on the subject and you have to keep AF active. With CF4-3, that means you hold down * as long as you're tracking the subject, and you keep the center focus point on the subject, and you hit the shutter button as the urge strikes you.
Personally I never focus and recompose when shooting action, because if you do you'll lose focus. If you want to compose off center, you have to use an off-center focus point to do it. I don't do that either, but some sports shooters do, even choosing their focus point on the fly. I'm not that quick (by a long shot).
You *can* focus and recompose for static shots though, such as sideline reaction shots etc. CF4-3 (or -1) comes in handy for that because you can effectively use AI servo just like one shot — hit * briefly and then get off it to lock focus, recompose and shoot.
Make sense? As I mentioned in the sticky, using * to activate AF takes some getting used to, but most (but not all) sports shooters prefer it once they get the hang of it.
The best single place to stand, assuming nobody objects, is behind the endline, just in from the corner. If the refs don't want you behind the endline then just park yourself on the touchline (that's soccer-speak for sideline) close to the corner, so you get them coming at your. You won't have line refs to knock you down in a U-6 game so you can get pretty close to the line. For six year olds you shouldn't be standing though, but sitting or kneeling on the ground. It's better to shoot from their level rather than down on them.
As for IS, I don't think it makes much difference one way or the other when you're shooting action, assuming you have sufficient shutter speed in the first place.
Nill
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