You need to take some steps before using Live View. This workflow prevents you from accidentally using Live View when you don’t mean to, thus potentially losing a shot, and it also helps ensure that you’ve made all the settings necessary to successfully use the feature efficiently. Here are the steps to follow:
Set focus to Manual. It’s a good idea to set the lens focus switch to the MF position, because most Live View pictures are taken using manual focus. Take this step now so you won’t forget that you’ll be doing the focusing yourself.
Choose a shooting mode. Select a Creative Zone shooting mode. Live View doesn’t work with any of the Basic Zone modes. You can choose A-DEP, but the 40D will behave as if you selected ...