With CF4 set to 1 or 3, AF starts when you press the * button and stops when you let off of the * button. (See my first response in this thread for a description of the difference between settings 1 and 3, which difference has to do with AE, not AF.) This means, among other things, that you can use AI servo much like one shot to focus and recompose. More importantly (to me at least), it also makes it a lot easier to keep AF tracking a subject while getting on and off the shutter, and also somewhat easier to "lead" the AF a bit before hitting the shutter, to avoid the dreaded AI servo first-frame-OOF syndrome.
With CF4 set to 2, AF still starts/stops via the half shutter press just like the default behavior, but pressing * (while still maintaining a half shutter press) will stop AF for as long as you hold down *. This has always seemed extremely convoluted to me, and personally I can't imagine ever using it, but I have talked with several people who swear by it.
In any event, if for some reason you want to spool up IS without restarting AF when you're set to CF4-1 or -3, you can do it with the half shutter press.
Clear?
Nill
Linear Mode
