Canon AT 1 - Manual (Page 75)

Concerning the Exposure (Shutter
Speed and Aperture Coupling)
In order to obtain the correct exposure, it
is necessary to correctly match the shutter
speed with the aperture. The shutter speed
and the aperture are the main factors in
controlling the amount of light which is
allowed to strike the film, and when they
change, the quality of the image upon the film
also changes.
Effects of Changing the Shutter Speed
The explanations below are pertinent to
photography with fast moving subjects or
when it is intended to produce impressionistic
pictures of movement. Depending on the
selection of the shutter speed, you can freely
control the expression of movement.
If, as in example A, the photo is taken at
a shutter speed of 1/1000 sec., the movement
will be frozen. If, as in example B, with the
same subject, the photo is taken at a shutter
speed of 1/60 sec. with a panning technique,
the movement is well expressed.
Panning is really quite a simple technique.
Hold the camera firmly and continue twisting
the upper part of your body while following