Canon A-1 - Manual (Page 59)

Setting the Lens for AE Photography
The first four AE modes described above,
i.e. shutter-speed priority AE, aperture
priority AE, programmed AE and AE flash
photography, are performed with full aper­
ture metering through the lens. This means
that the diaphragm is fully open during
metering for easiest viewing and opera­
tion.
For correct operation in these four full-
aperture metering AE modes, only an
FD lens can be used and the lens aperture
ring must be set to the “A” mark. To do
this, simply hold in the EE lock pin on the
lens while turning the aperture ring from the
minimum aperture to the “A” mark. This
can be done either before or after the lens is
mounted on the camera. At the “A” mark,
the aperture ring is locked and cannot be
turned
to any other setting. Reverse the
procedure to disengage the aperture ring
from the “A” mark.
3-