Originally Posted by
John Chardine
AF micro-adjust could be the answer but Ravi implied that this happens when AI-Servo is on, so it doesn't happen all the time. If it were a AF micro-adjust issue he would see the problem all the time.
Ravi- the conditions you describe are exactly the sort of occasion I might expect to see unstable air, shimmer, and soft images. Fog is an indication that you have air pockets whose temperatures are cold enough to be below the dew point. When the sun comes out it does not heat evenly and you will be shooting through pockets of cool and warm air. And by the way, to the naked eye the air might look perfectly clear to you but this does not mean you will not have a problem with shimmer. If you suspect a problem like this, the best way to test it is to mount your gear on a tripod and focus on a stable target like a tree trunk or fence post some distance away. Turn on Live view and set to maximum magnification and have a look. Shimmer/unstable air will cause the Live view image to shift in and out of focus completely on its own. The degree of shimmer problem depends on the amount of air you shoot through so take a look at your images and see of the closer ones are better than the ones farther away.