Canon PowerShot A 1000 IS User Manual page 76

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.
Con-firm 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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