Canon A-1 - Manual (Page 80)

3. Full Aperture Signal Pin
This pin transmits the maximum aperture of
the lens to the exposure meter to set the
meter coupling range automatically.
4. AE Switch Pin
When the “A” mark of the aperture ring is
set to the aperture index, the AE switch pin
comes out to insure that the lens, at this
setting, can be mounted only on cameras
designed for AE photography.
5. Reserved Pin
This pin is designed for use with additions to
Canon's camera system that may be develop-
ed in the future.
Unusable Lenses
Out of the several series of Canon lenses,
there are a few older individual lenses which
cannot be used on the A-1. Do not attempt
to mount the following five lenses:
FL 19mm f/3.5
FL 58mm f/1.2
R 58mm f/1.2
R 100mm f/3.5
FLP 38mm f/2.8
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