Canon AT 1 - Manual (Page 98)

Changing the Lens
FD lenses incorporate a safety mechanism
to prevent the Breech-Lock ring and the dia­
phragm 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 breech-Lock mount while
turning the Breech-Lock ring. Once this safety
mechanism has thus been cancelled, you can
see the diaphragm blades move when activated.
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.
Take notice that the following lenses
cannot be used on the AT-1 due to inter­
ference with the body signal pins. Using these
lenses will cause improper meter readings
and may cause damage to the camera.
FL 19mm f/3.5 R 50mm f/1.8
FL 50mm f/1.8 R 58mm f/1.2
FL 58mm f/1.2 R 100mm f/2
R 35mm f/2.5 R 100mm f/3.5