Canon A-1 - Manual (Page 37)

Aperture Scale on the AT Dial
The aperture scale on the AT dial is calibrat-
ed in f/stops ranging from f/22 to f/1.2. The
f/1.2 setting is indicated by a dot. The dial
click-stops at both full and half f/stops. It
will not turn past the largest and smallest
f/stops. The maximum aperture indicated on
the AT dial is large enough for any Canon
lens.
Under certain conditions in the aperture
priority AE mode, the maximum and mini-
mum apertures on the lens have priority over
the aperture set on the AT dial. For
instance, if the aperture you have set on the
AT dial is larger than the maximum aperture
of the lens in use, the camera will match the
shutter speed to the lens maximum aperture
rather than the aperture you have set on the
AT dial for correct exposure. Thus, if you
are using an FD 50mm f/1.8 lens with the
AT dial set to f/1.4, the camera will use
f/1.8 as the aperture setting. In this case, the
maximum aperture of the lens will be
displayed in the viewfinder when you take
an exposure preview regardless of the setting
on the AT dial. On the other hand, if you
have set an aperture on the AT dial smaller
than the minimum aperture of the lens, the
image will be exposed at the lens minimum
aperture but the shutter speed will not be
adjusted accordingly, resulting in over-ex-
posure. In this case, the aperture set on the
AT dial will be displayed in the digital read-
out.
Since the smallest aperture that can be set
on the AT dial is f/22, if you want to set an
aperture smaller than f/22, switch to
stopped-down AE and manually set the
aperture on the lens aperture ring.
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