Canon EOS Rebel T3i 18-55mm IS II Lens Kit - Manual (Page 107)

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You can lock the exposure when the area of focus is to be different from
the exposure metering area or when you want to take multiple shots at
the same exposure setting. 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 autoexposure
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.
* When the lens’ focus mode switch is set to <MF>, AE lock is applied at the center AF point.
.
A Locking the ExposureN
AE Lock Effects
Metering Mode
(p.102)
AF Point Selection Method (p.85)
Automatic Selection Manual Selection
q *
AE lock is applied at the AF
point that achieved focus. AE lock is applied at the
selected AF point.
wre
AE lock is applied at the center AF point.
COP
Y