Nikon D4 - Manual (Page 357)

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Virtual Horizon
When Viewfinder virtual horizon is selected for f3 (Assign Fn
button) > Press, pressing the Fn button displays a pitch and roll
indicators in the viewfinder. Press the button a second time to clear the
indicators from display.
Roll
Camera tilted right Camera level Camera tilted left
Pitch
Camera tilted forward Camera level Camera tilted back
The roles of the pitch and roll indicators are reversed when the camera
is rotated to take pictures in “tall” (portrait) orientation. Note that the
display may not be accurate when the camera is tilted at a sharp angle
forward or back. If the camera is unable to measure tilt, the amount of
tilt will not be displayed.