Canon AT 1 - Manual (Page 47)

Shutter and Aperture
The opening of the shutter letting light
in on the film is called an exposure. The
amount of light striking the film is controlled
by the lens aperture, while the length of time
that light is allowed to strike the film is
controlled by the shutter.
Shutter Speed Dial
The shutter dial is used to adjust the
shutter speed. It allows for speed settings in
the range of 2 seconds to 1/1000 of a second.
When the shutter speed is advanced to the
next larger number, the exposure time is cut
in half. The shutter speeds on the dial are
typically the reciprocals of the true shutter
speeds. For example, 125 and 250 on the dial
represent shutter speeds of 1/125 and 1/250
of a second. Only in the case of the orange
"2" is the shutter speed actually as indicated
on the dial, i.e., 2 seconds. The "B" (Bulb)
setting is used for long exposures where as
long as the shutter button is pressed down,
the shutter will remain open.
■ The shutter speed dial cannot be set to an
intermediate position.