Nikon Df User Manual page 285

Option
K Playback
None
A
Incompatible Options
If the option selected for Press can not be used in combination with the
option selected for Press + command dials, a message will be
displayed and whichever of Press or Press + command dials was
selected first will be set to None.
A
Viewfinder Virtual Horizon
When Viewfinder virtual horizon is selected for Custom Setting f4
(Assign Fn button) > Press, pressing the Fn button converts the
exposure indicator in the viewfinder to a roll display. Press the button
again to restore the indicator to its normal function.
Viewfinder
Note that the display may not be accurate when the camera is tilted at a
sharp angle forward or back. For a display that shows both pitch and
roll, use the Virtual horizon option in the setup menu (0 282).
Fn button performs same function as K button.
Select when using a telephoto lens or in other
circumstances in which it is difficult to operate the K
button with your left hand.
Pressing the button has no effect.
Camera tilted right
Description
Camera level Camera tilted left
U
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