I am currently running a Canon 70-300mm f4-5.6 IS on my 30D. Since going to Africa a few months ago I have been really getting into wildlife photography and have beenheaps from sites like this one.
Anyway, when I use AI Servo (most of the time with wildlife) I notice that the lens seems to be constantly hunting for focus. Its not hunting like in low light, just very small movements in and out of focus. To me it seems like it is moving one single increment each way on the focus motor, but I dont know if thats really the case.
Its really frustrating because at the long end, even at f8, the depth of field is narrow enough to make the slight movement in focus completely ruin the shot.
I have taken test snaps with the 300f4 and the 100-400 and neither of those exhibit the same behaviour. So is it just the difference in the focusing mechanism between these lenses or do I have a problem with my lens.