Nikon f2 - Manual (Page 18)

Frame Counter
The frame counter operates automatically to show how
many frames have been exposed. When the camera back
is opened for loading, the counter is reset to the “S”
(start) position, two frames before “0”. Once the
camera is loaded and the back closed, each film-advance/
shutter-release sequence will cause the frame counter to
advance one position until the maximum 20 or 36
frames have been exposed. The dial of the frame counter
is calibrated with dots (for odd numbers) and figures
(for even numbers), and with settings of “S,” “12,”
“20” and “36” in red.
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