Most of the old hacks will know this but I thought I would share with so many new comers to CiC
Using AE Lock with Back Button Focusing
I have been toying with the idea of setting my 40D for back button focusing - That is where the camera is configured to use the AE lock (*) button on the back of the camera for autofocus. However, after reading THIS I am not so sure it will fit my style of shooting. The main point that made me think again is this.
When the back button is configured to initiate autofocus (Menu C.Fn IV-1 - AE Lock/Metering + AF Start AND MNU C.FN IV-2 Enable) pressing the shutter halfway down locks the exposure setting so that you can meter off one area of the scene, recompose and focus on an other area without altering the exposure. That is fine but it is worth considering that if you wish to keep that exposure setting for the next shot you cannot take you finger off the shutter - you must release pressure to the half down shutter position again. A bit fiddly with cold or gloved hands.
And a question for the more tech minded; If I switch the AE lock from back button to shutter can I lock exposure in other camera modes than One Shot and evaluative metering. I say this because when the shutter is set to focus and hold the exposure setting (default) the camera only holds exposure in one shot evaluative metering modes. For any thing else including spot focus the metering is not locked by the depressing the shutter half down (you have to keep the back button (*) depressed). You would assume that when you switch buttons that the shutter behaves exactly like the AE Lock button but I cannot find any positive reference to confirm this.