Nikon PC NIKKOR 35mm f-2.8 - Manual (Page 10)

The PC-Nikkor is provided with a manual diaphragm
operable from f/2.8 to f/32 and a spring-loaded preset
ring to aid in setting the diaphragm quickly to the
shooting aperture; a total of eight click-stop positions,
at full-stop intervals, makes for precise placement of
the preset ring. Also, identical sets of f/numbers are
engraved on opposite sides of the front rim of the lens
to facilitate operation when the lens is used inverted.
To set the aperture, the following procedure is recom­
mended: Press the preset ring toward the camera body
and turn it until the f/number setting corresponding
to the desired shooting aperture is aligned with the
ring’s index dot; then release the ring and it will spring
outward and lock in place. Note that the preset ring,
once set, limits the travel of the aperture ring so that
the diaphragm can be set easily and quickly from the
full aperture viewing position to the shooting aperture
without the need for checking the scale during stop-
down operation.