Canon EF - Manual (Page 44)

BASIC OPERATION
26
Setting the Aperture Ring and the
CAT Switch
The amount of light that is allowed to strike the
film is determined by the lens diaphragm. This
amount of light is represented by "f" numbers or
f/stops engraved on the aperture ring which clicks
into place as it is rotated. The f/stop designations
indicate the opening formed by the diaphragm, and
this opening is called the aperture. On the standard
50mm lens, the engraved f/stops are 1.4, 2, 2.8, 4,
5.6, 8, 11, and 16. At f/1.4, the lens is at its
maximum aperture with the diaphragm wide open.
For this reason, f/1.4 is said to be a large f/stop even
though it is small in numerical value. And con­
versely,f/16 is the minimum aperture of the lens and
is a small f/stop. The relationship between successive
f/stops is arithmetic: as you close down the dia­
phragm, each f/stop indicates half as much light as
the preceeding one. When operating the aperture ring
manually, you may use any aperture setting. For
normal AE operation, the aperture ring must be set
at the green mark. Hold in the AE lock pin while
turning the aperture ring from f/16 (or f/22 on some
lenses) to the green "0."At this special setting, the
aperture ring is disengaged, allowing the camera to
adjust the lens diaphragm automatically to any
f/stop setting. Also, to make sure that the camera
selects the proper f/stop, the CAT switch must be set