Canon AT 1 - Manual (Page 93)

Stopped-Down Metering
When the AT-1 is used with Canon FD
lenses, photography can be performed with
match needle type full aperture metering.
Even when the lens automatic aperture lever
is locked in the manual position, FD lenses
should not be used on the AT-1 with stopped-
down metering. This will cause improper
meter readings. However, with the Canon
FL lenses and most accessories such as bellows,
extension tubes, or a microscope adapter, it is
necessary to take a stopped-down meter
reading. Stopped-down metering is performed
by pushing the stopped-down lever until it
locks with the main switch at ON, and
adjusting the shutter dial and/or the aperture
ring until the meter needle is aligned with the
aperture needle. Press the shutter button
and the photograph will be prefectly exposed.
If the lens should be mounted on the
camera with the stopped-down lever locked,
correct exposure will not be obtained. In this
case, a red warning mark by the stopped-down
coupling lever inside the camera body is
visible. After removing the lens, on the lower
part of the camera body, just below the