Canon EOS Rebel T3i EOS REBEL T3i / EOS 600D Instruction Manual - Page 53

A Full Auto (Scene Intelligent Auto) Techniques Recomposing the Shot Depending on the scene, position the subject toward the left or right to create a balanced background and good perspective. In the (Scene Intelligent Auto) mode, while you press the shutter button halfway to focus a still subject, the focus will be locked. You can then recompose the shot and press the shutter button completely to take the picture. This is called "focus lock". Focus lock is also possible in other Basic Zone modes (except Sports). Shooting a Moving Subject In the (Scene Intelligent Auto) mode, if the subject moves (distance to camera changes) during or after you focus, AI Servo AF will take effect to focus the subject continuously. As long as you keep aiming the AF point on the subject while pressing the shutter button halfway, the focusing will be continuous. When you want to take the picture, press the shutter button completely. 53