Nikon D300s - Manual (Page 372)

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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 D300S, in contrast, is approximately
23.6 × 15.8 mm, meaning that the diagonal picture angle of a 35mm
camera is approximately 1.5 times that of the D300S. To calculate the
focal length of lenses for the D300S in 35mm format, multiply the focal
length of the lens by 1.5 (for example, the effective focal length of a
24mm lens in 35mm format would be 36 mm when mounted on the
D300S).
Picture size (35mm format)
(36 × 24 mm)
Picture diagonal
(23.6 × 15.8 mm)
Picture size (D300S)
Picture angle (35mm format)
Picture angle (D300S)
Lens