Nikon D3000 - Manual (Page 176)

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Identifying CPU and Type G and D Lenses
CPU lenses can be identified by the presence of CPU contacts, type G and D lenses by a letter
on the lens barrel. Type G lenses are not equipped with a lens aperture ring.
CPU contacts Aperture ring
CPU lens Type G 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.