Canon A-1 - Manual (Page 79)

Lens Signal Coupling
There are several levers and pins at the rear
of an FD lens which transmit signals be-
tween the lens and the camera body. Usually
it is not necessary to know what they are,
but sometimes, when you must operate the
aperture manually for non-coupled acces-
sories for instance, it is. Just for your
information, we include explanations of
them here.
1. Aperture Signal Lever
With an FD lens, exposure metering is
performed through the lens at full aperture.
This is nice for you since you have a bright
viewfinder to focus and compose, but to
determine correct exposure, the camera
must know the effective aperture. For most
cameras, such as the Canon F-1, this lever
transmits the preset aperture on the lens
aperture ring to the exposure meter. It is
coupled to the lens aperture ring and moves
in proportion to its rotation. In AE
photography, however, the exposure meter
receives a signal directly from the camera.
2. Automatic Aperture Lever
This lever couples to the camera body to
stop the diaphragm down to the preset
aperture just before the shutter releases. This
is the lever that must be locked for manual
diaphragm control with a non-coupled acces-
sory.
Aperture Signal
Lever
Full Aperture
Signal Pin
Reserved Pin
Automatic
Aperture Lever
AE Switch Pin
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