Canon AT 1 - Manual (Page 97)

Lock for Manual Aperture Control (2)
There are some FD lenses with the manual
lock lever requiring a different procedure for
manual control setting. With these particular
lenses, the automatic aperture lever must be
turned fully counterclockwise while the
manual lock lever is brought to the "L"
position. Once this has been done, when the
lens is mounted on the camera, the diaphragm
blades will open or close by turning the
aperture ring. To revert from manual aperture
control, reset the manual lock lever at the
position of the white dot.
Lock for Manual Aperture Control When
Using the Macrophoto Coupler (3)
In close-up photography of high mag­
nification with a lens reversed on the Macro­
photo
Coupler, the automatic diaphragm
mechanism is not coupled, you must,
therefore, remember to close down the
diaphragm manually after having locked the
automatic
aperture lever in the manual
position as explained above in (1) and (2).
Then, fix the Macrophoto Hood on the lens
mount by turning the bayonet ring.