Canon AE-1 PROGRAM Instructions Manual page 52

4. Push in the stop-down lever until it locks.
Now, just by looking at your subject through
the viewfinder, you can see the range of
sharp focus.
It is impossible to push in the stop-down lever when
an FD lens is set at " A."
52
5. After checking the depth of field, unlock the
stop-down lever. Now turn the aperture ring
to the smallest number. Then turn it to the
largest number, press the AE lock pin and
return the aperture ring to "A."
Do not push in the stop-down lever before you
advance the film or the diaphragm will close
down only as far as the aperture used for the
previous exposure.
When an FD lens is mounted DIRECTLY on the
camera (with no accessories between) ,
NEVER take a shot before releasing the stop-
down lever or exposure may be incorrect. And
unless you want to make an exposure correc-
tion (p. 55) return the aperture ring to "A"
before shooting.