Canon A-1 Repair Manual page 24

the shutter speed is determined automatically to
respond t o f / l.8 for correct exposure.
In aperture priority AE mode, it is impossible to set
an aperture value of f / 27 or f/32 since they are not
on the dial.
If an aperture value smaller than the minimum aperture
value of the lens used is set, exposure value corres-
ponding to the set aperture value is displayed and
the exposure will be over if the shutter button is
pressed. (No warning in this case)
(Example)
Using a SOmm f/l.4 lens with minimum aperture of
f / 16, if the A/T dial is set to f/22, the view-
finder shows the exposure value calculated accord-
ing to f/22 and the photo will be overexposed by
one f / stop.
Both the scales of the f ilm speed setting dial and
those of the exposure compensation dial are calibrated
for every 1/3 step increment while the display is
calibrated in 1 / 2 step increments. If either dial is
changed by 1/ 3 step, it occurs that the display re-
mains the same or is changed by half a step . (Actual
exposure is correct since actual exposure control is
in 1/8 increments.)
"M" appears in the following cases:
a) Camera body and lens
a-l) When the lens is off "A" mark and is not
stopped down
a-2) When the shutter speed is set to "B"
b) With a Speedlite in use
b-l) When the lens is off "A" mark
b-2) When the Speedlite is in manual mode
b-3) Whe n the shutter s p eed is set to "B" (As an
exception, "M" does not appear when both the
lens and the Speedlite are in automatic mode
even though the shutter speed were set to "B".)
b-4) When the stopped-down lever is pressed.