Nikon D5 - Manual (Page 351)

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Calculating Angle of View (Continued)
The DX (24×16) angle of view is about 1.5 times smaller than the 35 mm
format angle of view, while the 1.2Ч (30Ч20) angle of view is about 1.2
times smaller and the 5 : 4 (30×24) angle of view is about 1.1 times
smaller. To calculate the focal length of lenses in 35 mm format when
DX (24×16) is selected, multiply the focal length of the lens by about
1.5, by about 1.2 when 1.2Ч (30Ч20) is selected, or by about 1.1 when
5 : 4 (30×24) is selected (for example, the effective focal length of a
50mm lens in 35 mm format would be approximately 75 mm when
DX (24×16) is selected, 60 mm when 1.2Ч (30Ч20) is selected, or
55 mm when 5 : 4 (30×24) is selected).