Nikon D7100 - Manual (Page 112)

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Autoexposure Lock
Use autoexposure lock to recompose photographs after using center-weighted
metering and spot metering to meter exposure. Note that matrix metering will not
produce the desired results.
1
Lock exposure.
Position the subject in the selected focus point
and press the shutter-release button halfway.
With the shutter-release button pressed halfway
and the subject positioned in the focus point,
press the A AE-L/AF-L button to lock exposure (if
you are using autofocus, focus will also lock;
confirm that the in-focus indicator (I) appears
in the viewfinder).
While exposure lock is in effect, an f indicator
will appear in the viewfinder.
2
Recompose the photograph.
Keeping the A AE-L/AF-L button pressed,
recompose the photograph and shoot.
Shutter-release
button
A AE-L/AF-L
button