Nikon D800 - Manual (Page 152)

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Autoexposure (AE) 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, confirm that the
I in-focus
indicator appears in the viewfinder).
While exposure lock is in effect, an AE-L
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