Canon AE-1 Manual page 27

Changing the Lens Earlier FD lenses (With inner chrome ring) incorporate a safety
mechanism to prevent the bayonet ring and the diaphragm blades from moving when the lens
is not mounted on the camera. To bypass this safety mechanism, press the lock pin in the top
recess of the bayonet mount while turning the bayonet ring. Once this safety mechanism has
thus been cancelled, you can see the diaphragm blades move when activated. Note: Later FD
lenses (those with the black ring) need not have to lock the chrome ring. Since FD lenses have
signal pins and levers which couple with the camera body, special care must be taken not to
damage them. One basic precaution is to always put the lens down facing down whenever you
must change lenses.
The following lenses cannot be used with the built-in meter because the extended rear part of
the lens will push in the lens speed adjustment pin on the camera body: FL 19mm f/3.5 FL
35mm f/2.5 FL 50mm f/1.8 FL 58mm f/1.2 R 35mm f/2.5 R 50mm f/1.8 R 100mm f/2.
Lens Signal Coupling Aperture Signal Lever
This lever transmits the actual f/stop to the exposure meter. It is coupled to the aperture ring
just the same as when the aperture ring is not set at the "A" mark.
Full Aperture Signal Pin This pin transmits the signal corresponding to the lens at the full
aperture opening.
Automatic Aperture Lever This lever closes down the aperture, coupled with the stopped-
down coupling lever.
When you are taking stopped-down meter readings, the manual
aperture control "M" signal above the aperture scale inside the
viewfinder flashes on and off only when the shutter release button is
depressed halfway.