Nikon D700 User Manual page 402

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Calculating Picture Angle
The D700 can be used with Nikon lenses for 35mm (135) format cameras.
If Auto DX crop is on (the default setting) and a 35mm format lens is
attached, the picture angle will be the same as a frame of 35mm film
(36.0 × 23.9 mm); if a DX lens is attached, the picture angle will
automatically be adjusted to 23.5 × 15.6 mm (DX format).
To choose a picture angle different from that of the current lens, turn Auto
DX crop off and select FX format (36 × 24) or DX format (24 × 16). If a
35 mm format lens is attached, the picture angle could be reduced by
1.5 × by selecting DX format (24 × 16), exposing a smaller area.
FX format (36 × 24) picture size (36.0 × 23.9 mm,
equivalent to 35mm format camera)
Picture diagonal
Lens
DX format (24 × 16) picture size
(23.5 × 15.6 mm, equivalent to DX
format camera)
Picture angle (FX format (36 × 24);
35mm format)
Picture angle (DX format (24 × 16); DX format)
The DX format (24 × 16) picture angle is about 1.5 times smaller than the
35mm format picture angle. To calculate the focal length of lenses in
35mm format when DX format (24 × 16) is selected, multiply the focal
length of the lens by about 1.5 (for example, the effective focal length of a
50mm lens in 35mm format would be 75 mm when DX format (24 × 16)
is selected).
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