if you root around in the settings you will see you can decouple the AF from the half-press on the shutter button, but a half press will still start the SR. I use the AF button on the back to focus and keep the shutter button half pressed to avoid any lag time waiting for the SR to get going. It's not so needed when shooting static objects but fr action shots it really helps a lot.
FYI, to focus you will need to either manual focus or use the AF button itself.
