Nikon SLRs Series Instruction Manual page 15

Turn the focusing ring until the two halves of the rangefinder image coincide to form a
single, sharp image. Focusing can also be done by setting the distance scale on the lens
barrel for the lensto-subject distance as measured or estimated. This is especially useful
when the camera must be prefocused to capture an elusive subject.
Focusing is always done at full aperture with Nikkor Auto lenses. This ensures a bright,
clear finder image for accurate focusing, viewing and framing. It also minimizes depth of
field so the image snaps in or out of focus distinctly.
Out of focus In focus