Nikon D3000 Reference Manual page 176

A
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
A
Lens f-number
The f-number given in lens names is the maximum aperture of the lens.
n
158
CPU lens Type G lens
Aperture ring
Type D lens