Nikon D100 Manual page 191

Picture Angle and Focal Length
A 35-mm camera has a diagonal picture angle approximately one-and-a-half
times that of the D100. When calculating the focal length of the lenses for
the D100 in 35-mm format, you will therefore need to multiply the focal
length of the lens by 1.5, as shown in the following table:
Picture angle
35-mm film camera
D100
35-mm film camera
D100
Calculating Picture Angle
The size of the area exposed by a 35-mm
camera is 24 36 mm. The size of the area
exposed by the D100, in contrast, is
15.6 23.7 mm. As a result, the picture
angle of photographs taken with the D100
differs from the picture angle for 35-mm
cameras, even when the focal length of
the lens and the distance to the subject
are the same.
Approximate focal length (mm) in 35-mm format
(modified for picture angle)
17 20 24
25.5 30 36
105 135 180
157.5 202.5 270
Lenses and Other Accessories
28 35
42 52.5
200 300
300 450
Lens
Picture angle (35-mm format)
50 60 85
75 90 127.5
400 500 600
600 750 900
Picture size (35-mm format)
(24 mm 36 mm)
Picture diagonal
Picture size (D100)
(15.6 mm 23.7 mm)
Picture angle (D100)
179