Canon AE-1 - Manual (Page 72)

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.
1 Effects of Changing the Shutter Speed
The explanations below are pertinent to
photography with fast moving subjects or
when it is intended to convey the feeling of
movement in a photograph.
If, as in example A, the photo is taken at
a shutter speed of 1/250 sec., the movement
will be frozen.
If, as in example B, with the same
subject, the photo is taken at a speed of 1/60
sec. though the subject is somewhat blurred,
movement is well expressed. It is only a
matter of aesthetics as to which of these
photographs is the best.
Depending on the selection of the shutter
speed, you' can freely control the expression
of movement.