DSLR Shots and Discussions

Originally Posted by 10MPlayer

To focus, the camera does a computer analysis of the light coming into it. When the image is out of focus the camera doesn't see much difference from one part of the image to the next. As it focuses it "sees" a sharper division between the bright areas and the dark areas and tries to maximize that effect.

When you look at a dark night sky there's not enough light. The camera can't find any contrasting objects to focus on. You need to turn autofocus off and wait 10 minutes or so for your eyes to adjust. Then you manually focus the camera. Of course you use a tripod and a remote release and don't bother if it's windy out.

Here's an easy to read article on autofocus... HowStuffWorks "How Autofocus Cameras Work"