Canon 40D - Manual (Page 95)

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Use AE lock 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 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.
* When the lens’ focus mode switch is set to <MF>, AE lock is applied at the
center AF point.
A AE LockN
AE Lock Effects
Metering Mode
AF Point Selection Method
Automatic Selection Manual Selection
q Evaluative
metering*
AE lock is applied at the AF
point that achieved focus. AE lock is applied at the
selected AF point.
w Partial metering
AE lock is applied at the center AF point.
r Spot metering
e Center-weighted
average metering