think its related to distance?
May well be right - think it works much better closer up. I took a test shot in a church in readiness for a christening on Sunday I have to cover - focused on some flowers in front of the altar and the rostrum behind was in focus. Ok, it was white flowers in front of a white altar but still doesn't explain the back focus...thing is I can't always replicate it. On the the other hand the 24-70 locked on (albeit at 2.8) without any problems. 

Its a terrific lens its so sad to hear about so many having issues with it working 






