Canon A-1 Instructions Manual page 73

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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
camera to a higher value. Usually this
is done by multiplying the normal ASA rating
by some multiple of two, say by two or four.
In this case,
film must be shot at the "pushed" ASA
or you will have to sacrifice one or the other
of a whole series of frames. Also, do not
forget to inform your developing lab of the
change or the whole film will be incorrectly
developed. This is only an introduction to
this technique. Not all films can be
"pushed" and not all developing labs will
handle them. Read more about "pushing"
the ASA in photographic literature and
know what you're doing before you try it.
This trick should not be confused with the
above which involves intentional exposure
correction for only one frame after which
the ASA dial should be reset to its original
value.
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