Canon A-1 - Manual (Page 67)

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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