Is this normal?

[fats]

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 been learning heaps 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.

[MarkChap]

Yep, completely normal.

The three modes on the canon are
Single Shot - Obtains focus once and will not re-focus until you push the button again
AI Focus - Obtains Focus expecting the subject to be still but is ready and waiting in case you or the subject move
AI Servo - Obtains focus but is expecting the subject to be moving, so is always re-adjusting focus,

I always use AI Focus, combined with back button focus, best of both worlds for me

[fats]

Thanks for the quick reply.

Does the hunting get better with better glass? It seemed much better with the L lenses to me.

[MarkChap]

Nope, and it is not really hunting

By telling the camera to use AI Servo, you are saying that the object I am photographing IS moving so I want you to track it constantly.
You will be moving even just the slightest bit and the Af system is correcting for that

[fats]

Oh, well maybe there is some confusion here then.

I know that the focus is going to track a moving object, but what mine is doing is moving in and out of focus by a noticable amount.

For example, I can have the cam on a tripod focusing on a static object and the focus motor will still be constantly moving.

Rather than finding focus and then tracking the object (which is static in this case) it constantly jumps in and out of focus. Shouldnt the motor only adjust when the object moves?