Canon F-1 Instructions Manual page 26

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The aperture ring of most Canon FD lenses is
equipped with click-stops for full and 1/2 f/stops, but
can be set between these click-stops.
The maximum aperture of a lens may not be one
included by the international standard. If it is not, the
second f/stop will not designate an aperture one-half as
small as the maximum f/stop.
The relation between the aperture and the amount of
light entering, using f/2 as the base, is as follows:
*
f/stops :
1.2 1.4 1.8 2 2.8 3.5
Exposure Ratio:
3 2 1.25 1 1/2 1/3 1/4 1/8 1/16 1/32 1/64 1/128
*
f/stops on the international scale are in heavy type.
Presetting of Aperture
Set the aperture ring
diaphragm will close to the set f/stop only for the
instant that the shutter is released. Thus, for example,
with FD lenses, the subject can always be seen through
the viewfinder at the full aperture opening even after
the f/stop has been set by the aperture ring.
The FD lens has only one aperture ring. When this lens is
mounted on an F-1 or FTb camera body, the diaphragm
can be operated manually by pushing the multi-purpose
lever. The aperture can then be closed down to any
desired f/stop by turning the aperture ring. When the
lever is reset to its original position, the diaphragm
returns to its maximum opening.
26
4 5.6 8 11 16
to the desired f/stop. The
22
Automatic
Aperture Lever