Nikon D750 DSLR Camera (Body Only) - Manual (Page 263)

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Non-CPU lenses can be used in 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 ( 0 433)
• 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 control panel and
viewfinder
• Flash level is adjusted for changes in aperture if the flash unit
supports AA (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
Non-CPU Lenses