Canon EOS Rebel XTI - Manual (Page 84)

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Use AE lock when you want to take multiple shots at the same exposure
setting or when the area of focus is to be different from the exposure
metering area. Press the button to lock the exposure, then recompose
and take the shot. This is called AE lock. It is effective for backlit subjects.
1
Focus the subject.
Press the shutter button halfway.
X The exposure setting will be
displayed.
2
Press the <A> button. (0)
X The icon lights in the viewfinder
to indicate that the exposure setting is
locked (AE lock).
Each time you press the button,
it locks the current auto exposure
setting.
3
Recompose and take the picture.
If you want to maintain the AE lock
while taking more shots, hold down
the button and press the shutter
button to take another shot.
A AE LockN
AE lock indicator
The AE lock effect will differ depending on the AF point and metering mode.
For details, see “AE Lock” (p.159).