Nikon D750 User Manual page 460

Calculating Angle of View
A
The D750 can be used with Nikon lenses for 35mm (135) format
cameras. If Auto DX crop is on (0 111) and a 35mm format lens is
attached, the angle of view will be the same as a frame of 35mm film
(35.9 × 24.0 mm); if a DX lens is attached, the angle of view will
automatically be adjusted to 23.5 × 15.7 mm (DX format).
To choose an angle of view different from that of the current lens, turn
Auto DX crop off and select from FX (36×24), 1.2× (30×20), and DX
(24×16). If a 35mm format lens is attached, the angle of view could be
reduced by 1.5 × by selecting DX (24×16) or by 1.2 × by selecting 1.2×
(30×20) to expose a smaller area.
FX (36×24) picture size (35.9 × 24.0 mm,
equivalent to 35mm format)
1.2× (30×20) picture size
(29.9 × 19.9 mm)
DX (24×16) picture size (23.5 × 15.7 mm,
Lens
equivalent to DX format camera)
Picture diagonal
Angle of view (FX (36×24); 35mm format)
Angle of view (1.2× (30×20))
Angle of view (DX (24×16); DX format)
The DX (24×16) angle of view is about 1.5 times smaller than the
35mm format angle of view, while the 1.2× (30×20) angle of view is
about 1.2 times smaller. To calculate the focal length of lenses in
35mm format when DX (24×16) is selected, multiply the focal length
of the lens by about 1.5 or by about 1.2 when 1.2× (30×20) is selected
(for example, the effective focal length of a 50mm lens in 35mm format
would be 75 mm when DX (24×16) is selected or 60 mm when 1.2×
(30×20) is selected).
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