Nikon D3X User Manual page 150

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.
F button
Sub-command dial
To unlock aperture, press the F button and
rotate the sub-command dial until the R
icons disappear from the displays.
SHOOT
CUSTOM
Z
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