Canon PowerShot SX400 IS Black Manual (en)

Helps avoid missing sudden photo opportunities, 
because the camera constantly focuses on subjects until 
you press the shutter button halfway.
Conserves battery power, because the camera does not 
focus constantly.
Shooting with the AF Lock
 Still Images 
The focus can be locked. After you lock the focus, the focal 
position will not change even when you release your finger from 
the shutter button.
Lock the focus.
With the shutter button pressed 
halfway, press the [ ] button.
The focus is now locked, and [
is displayed.
To unlock the focus, release the 
shutter button and press the [ ] 
button again. In this case, [
] is 
no longer displayed.
Compose the shot and shoot.