Drifting AF

[cspratt]

On my Canon 10D with the 300L F/4 IS and 1.4 extender the Auto Focus seems to be in a hunting mode. It appears to "lock-on" to subject then drifts just as I start to press the shutter for a shot. Seems that it is looking for something else to focus on. Anyone have similar experience with this combo?

[LzyPhotographer]

AF quits working at F5.6 .... There is a trick about taping the first 3 contacts to get AF back, but I personally don't do it. If you have really bright light, you may be more successful.