Canon IXUS 115 HS - Manual (Page 99)

99
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.
 The focus locks and % appears 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