Canon EOS 5D - Manual (Page 79)

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After achieving focus, you can lock the focus on a subject and
recompose the shot. This is called “focus lock.” Focus lock works only
in the One-Shot AF mode.
1
Set the Mode Dial to any setting
except <1> (Full Auto).
2
Select the desired AF point.
3
Focus the subject.
Move the AF point over the subject
and press the shutter button halfway.
4
Keep pressing the shutter button
halfway and recompose the
picture as desired.
5
Take the picture.
Focusing an Off-Center Subject
If the AF mode is AI Servo AF (or AI Focus AF set to Servo mode), focus
lock will not work.