Nikon D1 - Manual (Page 77)

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Depth-of-Field Preview
To check the depth of field for the aperture setting in the current exposure
mode, hold down the depth-of-field preview button.
The lens will be stopped down to the aperture selected
by the camera in programmed-auto or shutter-priority
auto exposure modes, or to the value selected by the
user in aperture-priority auto or manual modes. The
view through the viewfinder will give you an approxi-
mate idea of the depth of field that can be obtained at
the current aperture.
Focal Plane Position
The position of the focal plane inside the camera is indicated by a mark on the
camera body.
The distance between the camera and the subject
should be measured from this mark whenever distance
is measured manually. The distance between the lens
mounting flange and the focal plane is 46.5 mm (1.83˝).
Camera Settings: Depth-of-Field Preview/Focal Plane Position