I'm really confused as to how to properly use back button focus. I thought I had set it correctly in the custom functions (IV, #2), but it does not focus when I press the * button. I still have to press the shutter halfway for the lens to focus and then I hear the confirmation "beep". I thought the focus was supposed to happen when I press the * button. on #3 neither * or shutter buttons do anything. I read the Canon article but it only confused me more!