Nikon D3 - Manual (Page 154)

128
Z
❚❚Aperture Lock
To lock aperture at the selected value, press the F button and
rotate the sub-command dial until R icons appear in the
viewfinder and the top control panel.
To unlock aperture, press the F button and
rotate the sub-command dial until the R
icons disappear from the displays.
F button Sub-command dial
SHOOT
CUSTOM