Canon AT 1 - Manual (Page 94)

mirror, this stopped-down coupling lever
becomes visible, as does the red mark in the
case described above.
The Extension Tube FD 25 and FD 50
especially designed each for the FD 50mm
and FD 100mm macro lenses should be used
with full aperture metering. In this case, depth
of the field can be assured in the viewfinder
by pressing in the stopped-down lever.
Manual Aperture Control
When accessories requiring manual
aperture control are used between the camera
body and a lens, lock the lens automatic
aperture lever in the manual position before
mounting the lens.
Lock for Manual Aperture Control (1)
For manual aperture control, push the
automatic aperture lever counterclockwise
until it stops and locks. When accessories such
as extension tubes are attached to a lens that
has been set for manual control, the dia­
phragm blades of the lens open or close as the
aperture ring is turned. To revert from manual
control, reset the automatic aperture lever to
its original position.