I believe it's a fn setting on the camera that allows you to auto focus using one of the back buttons on the camera rather than the shutter button. How to do so is in the manual.
That's Custom Function 04 on the 20D I love it but some people hate it. If you set the function to 1 on the 20D you use the AE lock button on the back to focus. That way you can focus on your subject and then change change your composition and shoot away with out worrying about your focus point changing each time you push the shutter button.
So you hold the ae lock button with your thumb and blast away not having to mess about holding shutter button half way sounds good will give it a go if possible with 10D
That's Custom Function 04 on the 20D I love it but some people hate it. If you set the function to 1 on the 20D you use the AE lock button on the back to focus. That way you can focus on your subject and then change change your composition and shoot away with out worrying about your focus point changing each time you push the shutter button.
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Thanks Rberg & Khaos