Canon S90 用户手册

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页码 179
82
While you keep the shutter button pressed halfway, the focus and exposure 
are locked. You can now recompose and shoot. This is called Focus Lock.
Focus.
Center the subject you want in focus, and 
press the shutter button halfway.
Confirm that a green AF Frame appears 
around the subject.
Recompose.
Keep the shutter button pressed halfway 
and recompose the scene.
Shoot.
Press the shutter button fully.
Changing the Composition with the 
Focus Lock