You can temporarily interrupt Live View mode to allow the EOS 40D to use autofocus. Because the step takes a few seconds, you’re better off using autofocus when the camera is mounted on a tripod. If you handhold the 40D, you may displace the point of focus achieved by the autofocus system. Canon also recommends that you use One Shot focus and center the focus point. You can use AI-Servo and automatic or manual focus point selection, but if the focus point doesn’t coincide with the subject you want to focus on, you’ll end up with an out-of-focus image. Just follow these steps.
Make sure the focus switch on the lens is set to AF.
Activate autofocus using C. Fn. III-06 AF During Live View Shooting. Choose 1 ...