Canon AE-1 Instructions Manual page 60

Film Plane Indicator
This mark i s engraved on the top of the
camera between the film rewind crank and
the battery check button, just to the left of
the pentaprism, to indicate the exact position
of the film plane. The distance scale on the
lens shows subject distances measured from
the film plane indicator. This mark is not used
in general photography, but in close-ups and
macrophotography i t is often used to obtain
the exact subject distance.
Scales on the Lens
Aperture Scale
The aperture of the lens is the opening of
the diaphragm blades, like the iris of the
human eye. It controls the amount of light
passing through the lens to the film surface.
The flnumber is a numerical expression
of the effective aperture. I t is obtained by
dividing the focal length of the lens by the
diameter of the effective aperture. When the
flnumber is set one scale gradation higher, the
lens allows in half the light it would at the
previous gradation. Intermediate settings of
the aperture scale can be used, too. In some
lenses, the flnumber setting one gradation
higher than the first flnumber setting does not
necessarily allow only half the amount of light
of the previous setting through the lens to
expose the film a s i s the case at the other
settings. This should be taken into con-
sideration when necessary.
The aperture ring usually has gradations
marked as follows, taking f/2 as a basic
unit: