Nikon D810 - Manual (Page 371)

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Virtual Horizon
When Viewfinder virtual horizon is selected for f4 (Assign Fn
button) > Press, pressing the Fn button displays a pitch and roll
indicators in the viewfinder. Press the button a second time to return
to clear the indicators from display.
Roll
Camera tilted right Camera level Camera tilted left
Pitch
Camera tilted forward Camera level Camera tilted back
Note that the display may not be accurate when the camera is tilted at
a sharp angle forward or back.