Nikon D5 - Manual (Page 265)

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Non-CPU Lenses
Non-CPU lenses can be used in exposure modes A and M, with
aperture set using the lens aperture ring. By specifying lens data
(lens focal length and maximum aperture), the user can gain
access to the following CPU lens functions.
If the focal length of the lens is known:
• Power zoom can be used with optional flash units
• Lens focal length is listed (with an asterisk) in the playback photo
info display
If the maximum aperture of the lens is known:
• The aperture value is displayed in the top control panel and
viewfinder
• Flash level is adjusted for changes in aperture if the flash unit
supports qA (auto aperture) mode
• Aperture is listed (with an asterisk) in the playback photo info
display
Specifying both the focal length and maximum aperture of the lens:
• Enables color matrix metering (note that it may be necessary to
use center-weighted or spot metering to achieve accurate results
with some lenses, including Reflex-NIKKOR lenses)
• Improves the precision of center-weighted and spot metering
and i-TTL balanced fill-flash for digital SLR