Canon AE-1 - Manual (Page 99)

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 f/number is a numerical expression
of the effective aperture. It is obtained by
dividing the focal length of the lens by the
diameter of the effective aperture. When the
f/number 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 f/number setting one gradation
higher than the first f/number setting does not
necessarily allow onfy half the amount of light
of the previous setting through the lens to
expose the film as is 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:
Brightness
(f/stop)
Ratio
1.2 1 . 4 « 2.8 4 5.6 11 16
; 1/2 1/4 1/8 1/16 1/32 1/64