Canon EF - Manual (Page 46)

at the "NORMAL'' position.
* If the preceeding discussion seems complex, keep
in mind the following definitions:
An f/stop is a number used to represent the amount
of light which is allowed to pass through the lens.
The diaphragm is the mechanical iris inside the lens
which is opened or closed according to the setting of
the aperture ring.
The aperture is the hole or opening formed by the
diaphragm blades.
Gr Selecting a Shutter Speed
The amount of light striking the film is controlled by
the lens diaphram, whereas the length of time that
light is allowed to strike the film is controlled by the
focal plane shutter. The shutter consists of two
opaque "curtains" which travel over the opening and
allow the light coming through the lens to reach the
film. The length of time the curtains remain open
determines the exposure time for your picture. On
the shutter speed dial, shutter speeds from 1/1000 —
1 sec., and B, are marked in white (with the
exception of the orange "125" for electronic flash
synchronization), while the speeds from 2 — 30
seconds are marked in yellow. To set the shutter
speed, rotate the dial in either direction until the