Nikon D800 - Manual (Page 145)

En_03
121
Z
A
Non-CPU Lenses ( 0 371, 374)
Use the lens aperture ring to adjust aperture.
If the maximum aperture of the lens has been
specified using the Non-CPU lens data item
in setup menu (
0 213) when a non-CPU lens is
attached, the current f-number will be
displayed in the viewfinder and control panel,
rounded to the nearest full stop. Otherwise
the aperture displays will show only the
number of stops (F, with maximum aperture displayed as F
A) and the
f-number must be read from the lens aperture ring.