Nikon D750 DSLR Camera (Body Only) - Manual (Page 170)

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Metered Area
In spot metering, exposure will be locked at the value metered in a
4-mm (0.16 in.) circle centered on the selected focus point. In center-
weighted metering, exposure will be locked at the value metered in a
12-mm circle in the center of the viewfinder.
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Adjusting Shutter Speed and Aperture
While exposure lock is in effect, the following settings can be adjusted
without altering the metered value for exposure:
Mode Setting
P Shutter speed and aperture (flexible program; 0 89)
S Shutter speed
A Aperture
The new values can be confirmed in the viewfinder and control panel.
Note that metering can not be changed while exposure lock is in
effect.
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See Also
If On is selected for Custom Setting c1 ( Shutter-release button AE-L ,
0 336), exposure will lock when the shutter-release button is pressed
halfway. For information on changing the role of the A AE-L/AF-L
button, see Custom Setting f4 ( Assign AE-L/AF-L button , 0 361).