Canon 40D Live View Auto Focus
Canon 40D is one of the first cameras in the mid range professional lineup of Canon DSLRs to include live view functionality. However, there isn’t a way to autofocus on your subject without turning live view off. You can set focus mode to MF to manually focus on your subject. For better manual focusing, you can zoom in on the subject while on live view by pressing the reduce/enlarge buttons on the top right part of the back of the camera which are accessible by the right thumb and then adjusting the lens focus dial to get a good focus.
That being said, the closest live view auto focusing the camera has to offer is by the use of AF button. You can enable this from the menu by going to your cameras custom function settings. Select C.FnIII: Auto focus/Drive from there and change option 6 (Auto Focus/Drive AF during Live View shooting) to 1:Enable. During live view shooting now, this would interrupt the live view image display and focus on the selected rectangle for as long as you hold on to the AF-ON button.