Nikon D90 - Manual (Page 248)

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Compatible Lenses
CPU lenses (particularly types G and D) are recommended for use with the D90. 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.
A Calculating Picture Angle
The size of the area exposed by a 35mm camera is 36 × 24 mm. The size of the area exposed by
the D90, in contrast, is 23.6 × 15.8 mm, meaning that the picture angle of a 35mm camera is
approximately 1.5 times that of the D90. The approximate focal length of lenses for the D90 in
35mm format can be calculated by multiplying the focal length of the lens by about 1.5.
A Lens f-number
The f-number given in lens names is the maximum aperture of the lens.
CPU contacts Aperture ring
CPU lens Type G lens Type D lens
Picture size (35mm format)
Lens
(36 × 24 mm)
Picture diagonal
Picture size (D90)
(23.6 × 15.8 mm)
Picture angle (35mm format)
Picture angle (D90)