Nikon Df - Manual (Page 340)

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Recognizing CPU and Type G, E, and D Lenses
CPU lenses (particularly types G, E, and D) are recommended, but note
that IX-NIKKOR lenses can not be used. CPU lenses can be identified by
the presence of CPU contacts, type G, E, and D lenses by a letter on the
lens barrel. Type G and E lenses are not equipped with a lens aperture
ring.
CPU contacts Aperture ring
CPU lens Type G/E lens Type D lens
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Lens f-number
The f-number given in lens names is the maximum aperture of the lens.