Canon AT 1 - Manual (Page 41)

Checking Film Winding
Operate the film advance lever while
watching the film rewind knob. If it rotates,
the film is properly loaded. If the rewind
knob does not rotate, open the back cover
and load the film again from the start.
Setting the ASA Film Speed
After loading the film, set the ASA film
speed according to the ASA speed of the film
in use. To set the ASA, first push the film
advance lever out to its 30° stand-off position
away from the camera body, then gently lift
up the ASA ring around the shutter dial and
rotate it in either direction until the proper
number is aligned with the green index mark.
ASA is a numerical rating of a film's sensitivi­
ty to light. A higher ASA number indicates a
faster film which is more sensitive to light. On
the other hand, a lower ASA number indicates
a slower film which is less sensitive to light.
The ASA rating recommended by the manu­
facturer is printed on the film box, e.g.,
ASA 100.