Canon EOS Rebel T3i 18-55mm IS II Lens Kit - Manual (Page 53)

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Depending on the scene, position the subject toward the left or right to
create a balanced background and good perspective.
In the <
A> (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 <
5> Sports).
In the < A> (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.
A Full Auto (Scene Intelligent Auto) Techniques
Recomposing the Shot
Shooting a Moving Subject
COP
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