Nikon D800E User Manual page 12

Aperture(exposure modesR and/I): Press the selected button and
rotate the sub-command dial until [] icons appear in the
viewfinder and the control panel.
Fnbutton Sub-command dial
To unlock aperture, press the button and rotate the sub-
command dial until the [] icons disappear from the displays.
[] See Also
Use Custom Setting f7 (Shutter spd & aperture lock; _ 316) to keep
shutter speed and/or aperture locked at the selected values.
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