I purchased a 60d about 1.5 years ago and have gotten into wildlife and bird photography (also shoot my son's high school baseball). I purchased a new 400mm 5.6 about a month ago and am getting used to it. I've got more birding keeper photos with the 400 in the last month than I have in the last eighteen months. I used to use my 70-200 f4 (non-is), occasionally with a 1.4II.

Most of my roughly 1000 photos with the 400mm have been in focus if I hit my target (center point), often aimed at the birds eye. This morning I went to a new spot and found a few good opportunities at waterfowl I have not seen yet. I took about 100 shots, mostly of swimming ducks and a few in flight. It was drizzling on and off all morning, overcast and not ideal by any means. While shooting I thought to myself these might be my best photos yet. When I got home, I was very disappointed. It appears that most of the shots were out of focus, despite the fact that I hit my target (eyes) on many occasions.

Can a lens work fine for a month then have problems with auto focusing for no apparent reason? I assume I cannot do anything to correct this and may have to send the lens to Canon if the problem persists. Anyone have any ideas how a lens can appear fine for about a thousand photos then act up out of the blue? I will try another lens tomorrow and see if the problem is the lens or the body for starters.

Please share your thoughts.