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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 developed 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
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