Nikon D6 Reference Manual page 432

•
Entering both the focal length and the maximum aperture
allows use of:
- color matrix metering (Note that color matrix metering may
not produce the desired results with some lenses, including
Reflex-NIKKOR lenses. In these cases [Center-weighted
metering] or [Spot metering] can be used instead.)
•
Entering both the focal length and the maximum aperture
improves the precision of:
- [Center-weighted metering],
- [Spot metering], and
- i-TTL flash control
D
Teleconverters and Zoom Lenses
•
If the correct focal length is not available, for example because you are
using a teleconverter or zoom lens, choose the next largest value.
•
The maximum aperture for teleconverters is the combined maximum
aperture of the teleconverter and the lens.
380
Using Non-CPU Lenses (Non-CPU Lens Data)