Once you’ve enabled Live View, you can continue taking pictures normally through the 40D’s viewfinder. When you’re ready to activate Live View, press the Set button. The mirror will flip up, and the sensor image will appear on the LCD.
Here are some things you should keep in mind when Live View is active:
Manual focus. You must focus manually, unless you use the optional Live View interrupt/autofocus feature. Later in this section, I’ll provide instructions for using MF with Live View.
Exposure lag. If you reframe your photo, there is a lag before the exposure is set properly for the new composition. If you take a picture before the exposure has been re-calculated, you may get an under- or overexposed photo.
Metering mode ...