Canon IXUS 120 IS - Manual (Page 82)

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The focus can be locked. After locking the focus, the focal distance will not
change even when you release your finger from the shutter button.
Lock the focus.
● Keep the shutter button pressed halfway
and press the
q button.
X The focus locks and % is displayed on
the screen.
● If you release your finger from the shutter
button and press the
q button again, %
will disappear and the focus will unlock.
Compose the scene and shoot.
Shooting with the AF Lock