The IS has 2 modes type 1 and 2 the later is for action shots were you are panning and type 1 for still or single shots, or you can turn it off completely. I leave mine on type 2 most of the time for action shots (nature) but for racing shots I have been told to shut it off completely. I will get a chance to test both at the Daytona 24 hr race the end of this month. I used the 2x type ll on my 100-400 lens and was not happy with it, first I lost AF even though it was = to f11 2nd the standard focus screen on the 1d is a matte type unless you were in bright light it was hard to manually focus I have since ordered a split type focus screen . I found even off a tripod there was a difference in image quality so I returned the converter and bought a speedlight ex 550 flash instead. Randy