Canon PowerShot A 2000 IS User Manual page 76

Changing the Composition to Shoot (Focus Lock)
While you press the shutter button halfway, focus and exposure are fixed.
You can now recompose and shoot. This is called focus lock.
Focus on a subject.
Center a subject at the same focal
distance in the screen, and press the
shutter button halfway.
Confirm that the AF Frame on the subject
is green.
Recompose.
Hold the shutter button pressed halfway
and recompose the scene.
Shoot.
Press the shutter button fully.
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