Canon AT 1 - Manual (Page 108)

Canon Speedlite 155A
When the Speedlite 155A is used with the
AT-1, it is not necessary to set the shutter
speed on the camera as is the case with
ordinary electronic flash units. At any shutter
speed except "B", the shutter speed is
automatically adjusted to the X synchroni­
zation speed of 1/60 sec. at the time the
155A is charged. To perform automatic flash
photography, set the prescribed f/stops on the
lens manually.
As the flash is automatically controlled to
the given aperture value by the light reflected
back from the central part of the subject, the
influence of the background is greatly
reduced. Thus, the 155A offers more appro­
priate flash exposure rather than conventional
flash units.