Canon A2000 IS - Manual (Page 76)

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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.
Changing the Composition to Shoot (Focus Lock)