Canon A-1 - Manual (Page 79)

Programmed AE
When the AE mode selector is set to Tv and
the AT dial turned to “P” on the shutter
speed scale, the camera is set for program­
med AE. Correct exposure, both aperture
and shutter speed, is automatically set by
the camera according to the brightness of
the subject and a programmed set of combi­
nations of apertures and shutter speeds.
Metering control is done steplessly from a
combination of the highest shutter speed
and minimum aperture all the way to slower
shutter speeds and larger apertures.
Under some circumstances, the camera be­
haves as if it were set for aperture priority
AE. For example, if lighting conditions are
so low that an aperture larger than the
maximum aperture of the lens would be
necessary, the maximum aperture remains
fixed, and the camera’s electronic control
automatically shifts to slower shutter speeds
until the shutter speed is balanced against