Canon S90 用户手册

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页码 179
86
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.
 and the MF indicator appear. You can 
check the distance to the subject.
Compose the scene and shoot.
Shooting with the AF Lock