Canon EOS Rebel XT - Manual (Page 92)

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AE lock enables you to lock the exposure at a different place from the
point of focus. By locking the exposure, you can recompose the shot
while maintaining the same exposure setting. This is called AE lock. It is
effective for backlit subjects.
1 Focus the subject.
¡ Press the shutter button halfway.
s The exposure setting will be displayed.
2 Press the <
A> button.
(
0)
s The <
A> icon lights in the viewfinder
to indicate that the exposure setting is
locked (AE lock).
¡ Each time you press the <
A> button,
it locks the current exposure setting.
3 Recompose and take the picture.
¡ If you want to maintain the AE lock
while taking more shots, hold down
the <
A> button and press the shutter
button to take another shot.
A AE LockN
AE lock indicator
¡ If the AF mode is One-Shot AF or AI Focus AF (when not AI Servo AF) and
the metering mode is <
q> (Evaluative), pressing the shutter button halfway
will automatically set AE lock at the same time when focus is achieved.
¡ The AE lock effect will differ depending on the AF point and metering
mode. For details, see “AE Lock” (p.153).