Has anyone else had any problems with the Canon Mark IIN autofocus. Mine keeps coming out of adjustment. The camera will say it's in focus, but you have to turn the focusing ring about a quarter inch counter clockwise for it to actually be in focus. I thought it was the lenses at first but they work great on my other Mark II N. I sent it back to Canon and they adjusted the AF, I didn't have it back 5 minutes and it went out again. Any suggestions on how to deal with Canon?