Just had a random question pop into my head so I thought I would ask. The kit lens that comes with the 40d (28-135mm) isn't the fastest auto focusing, but not the slowest. I am going to a racetrack next weekend and I fear the auto focus isn't fast enough so I can try to manually focus but then all my shots may be slightly off. Do you think if I set it to just focus on the center AF point it will focus faster than if I had it set to all AF points? Thanks.