Excerpt from Scott Kelby's Digital Photography book, Chapter 3, The Truth About Lenses—Tips for Changing Lenses
Tips for Changing Lenses
If you own several lenses, you probably change lenses regularly when you are out shooting. Here are a few tips. First, you usually do not need to turn off the camera when changing a lens.
There are purists online who claim that as long as the sensor is active, it also attracts dust and so on, but I do not know a single professional who actually turns off the camera before changing a lens.
Dust is a problem, of course. When changing a lens, never point the camera's open body upward, because that lets dust enter the camera. You would be asking for trouble. It is better to point the open side of the camera downward.
If you are in a dusty environment (the Sahara just after a sandstorm, for example), do not change lenses at all. Wait until you are somewhere with clean air. Try to leave your camera body open and unprotected for as little time as possible because of that dust. So do not take five minutes to change lenses. Take one off and put the other one on. You do not need to rush (or you might drop something), but do not move at a snail's pace either.
When Your Autofocus Suddenly Stops Working
First check whether you accidentally turned it off. If it is still switched on, remove the lens and immediately mount it again. This trick has gotten me out of trouble many times.
