Yogi Bear wrote in post #14163419
Sometimes, AF problems can be caused by dust on the AF sensor. The sensor is located in the bottom of the mirror box.
Remove the lens and activate Mirror Lock Up. Turn the camera upside down and blow off the AF sensor with a rocket blower. Replace lens and deactivate MLU.
It's a long shot but it has been known to work.
I posted this in the other thread already, so sorry for the dupe, but I hope this helps...
Kinda related to what Yogi Bear just said:
A few months ago, I was having a lot of issues with intermittent focus hunting on my 50D. There was no discernible pattern I could identify, and it happened with all three of my primes (28, 85, and 200). On the forum, someone suggested to look for debris on the focus screen; stating that the AF system somehow uses the focus points in the viewfinder to compute focal distance. (I still don't know if this is true or not) That said, I took a look through my viewfinder, and there was a big 'ole stinking hair cutting directly across the center focus point. Normally, I have a fair amount of debris inside the body because I change lenses quite a bit outdoors and I usually blow the body out every couple of weeks. So, having a hair in the viewfinder is something I usually disregard to the point where I don't even notice it anymore.
Anyways, I pulled the focus screen out. Blew on it, re-installed it, powered up, and camera was working properly again.
To this day, I'm not sure if the hair on the focus screen was actually the cause of the issue, or if it was just one of those "oddball" things that corrected itself or just my general ignorance.
Anyway, i hope my post helps.