Nikon D6 Reference Manual page 1100

A
Compatible Non-CPU Lenses
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Specifying the lens focal length and maximum aperture using the [Non-
CPU lens data] item in the setup menu allows many of the features
available with CPU lenses, including aperture value display and color
matrix metering, to be used with non-CPU lenses. If the focal length and
maximum aperture are not provided and [Matrix metering] is selected
for metering, [Center-weighted metering] will be used instead.
•
Aperture must be set using the lens aperture ring. If the maximum
aperture is not provided using [Non-CPU lens data], the aperture
displays in the camera top control panel and viewfinder will show the
number of stops from maximum aperture and the actual aperture value
must be read off the lens aperture ring.
1048 Compatible F Mount Lenses