Nikon D3S User Manual page 396

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Picture Angle and Focal Length
The D3S can be used with Nikon lenses for 35mm (135) format cameras.
If Auto DX crop is on 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 from FX (36×24), 1.2× (30×20),
DX (24×16), and 5 : 4 (30×24). If a 35mm format lens is attached, the
picture angle 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, or the aspect
ratio could be changed by selecting 5 : 4 (30×24).
Lens
n
370
FX (36×24) picture size (36.0 × 23.9 mm,
equivalent to 35mm format camera)
1.2× (30×20) picture size
(23.5 × 15.6 mm, equivalent to DX
5 : 4 (30×24) picture size
Picture angle (FX (36×24);
Picture angle (1.2× (30×20))
Picture angle (DX (24×16); DX format)
Picture angle (5 : 4 (30×24))
(30.0 × 20.0 mm)
DX (24×16) picture size
format camera)
(30.0 × 23.9 mm)
Picture diagonal
35mm format)