Well...I think I may be having my first issue with back focusing. I must confess I didn't really know what this meant until I got home from shooting about 1200 pics of our local high school state championship game this evening.
I recently bought a used (<1000 shots) 30D and a lightly used 70-200/2.8.
About 1/2 of my shots appeared to have suffered from back focusing issues. I don't think it is user error b/c I've shot hundreds of football games and other sporting events with other gear and have never had a problem "hitting" my target.
In these shots, my moving subject is blurred and everything behind the subject is tack sharp. My subject (most of the time) is squarely in the middle of the frame and I used center-point AF.
Questions:
1.Is this problem with the lens or the camera body?
2.What is the next step in getting this fixed?
3.How can I test it to be sure it is, in fact a BF issue?
1. Try a few other lenses on your body. If they all work OK, it's the 70-200. Or, try a different body with the 70-200, and so on. Tough to tell, if you only have one body and one lens, though.
2. Send the offending item to Canon, they'll be able to fix it.
3. Sounds like you've already sufficiently tested the problem.
The problem can be with any combination of user, lens and body. It need not be one component that is at fault.
I take you did actually use AI Servo mode. Otherwise you'll get heaps of backfocus problems
You need to try the camera with known good lenses, and the lens with a know good camera, in order to isolate the problem. However, be aware that it may be inconsistent. e.g. it can vary with the lighting level, focal length, focus distance, etc.
If you cannot do that then send the lot in to Canon for service along with a detailed description of the problem. Canon will not match the lens to the camera - they'll match each one to independent, known standards. However, fixing the wrong one won't help you.
