Nikon D810 - Manual (Page 151)

127
Aperture (exposure modes g and h): Press the selected button and
rotate the sub-command dial until F icons appear in the
viewfinder and the control panel.
To unlock aperture, press the button and rotate the sub-
command dial until the F icons disappear from the displays.
Fn
button Sub-command
dial
A
See Also
Use Custom Setting f7 (Shutter spd & aperture lock; 0 350) to keep
shutter speed and/or aperture locked at the selected values.