Canon 650D - Manual (Page 61)

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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 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 mode, if the subject moves (distance to camera changes)
while or after you focus, AI Servo AF will take effect to focus the subject
continuously. (The beeper will continue beeping softly.) 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 Techniques (Scene Intelligent Auto)
Recomposing the Shot
Shooting a Moving Subject
COP
Y