Canon A-1 - Manual (Page 127)

Changing the ASA Setting
There is another way to correct exposure
which can be used whether you are in the
manual or the AE mode. That is to change
the ASA film speed setting on the camera. A
film with an ASA rating twice that of
another film requires only half the amount
of light for correct exposure as compared
with the other film. For instance, if you
have an .ASA 200 film loaded, you can make
an exposure correction equal to closing the
aperture one f/stop or raising the shutter
speed one step by changing the ASA setting
on the camera to ASA 400.
Other useful tricks can be performed by
changing the ASA setting. You may be
stuck, for instance, indoors without flash.
Even with ASA 400 film loaded in your
camera, you may be forced to use undesira­
bly slow shutter speeds. You may solve
this by "pushing" the ASA setting on the