Specifications

Shutter-speed priority AE mode: A number equal to or
smaller than the lens' maximum aperture blinks. Choose a
slower shutter speed so that the aperture stops blinking.
Shutter-speed priority AE mode: When 2 sec. is set on the shutter speed selector
dial and a number equal to or smaller than the lens' maximum aperture blinks; or
Programmed AE mode: When the lens' maximum aperture blinks, you can either: a)
Use flash or other additional light or b) Use a film with a higher ISO/ASA rating.
Note: With the shutter button pressed halfway, a red "M" LED
appears whenever you remove the aperture ring of the lens from "A"
or when you mount a non-FD lens. It warns you that exposure will
not be automatic.
14. Exposure Control
Taking a picture is a matter of letting light fall on the film under controlled conditions.
This is called exposure. When you press the shutter button, some blades (called a
diaphragm) inside the lens shift to form an opening called the aperture. Almost
simultaneously, the first shutter curtain starts to move inside the camera. A second
shutter curtain follows it after a fixed interval which you control with the shutter speed
selector dial. The amount of light that exposes a frame depends on the shutter speed
and the size of the aperture.
For the same exposure, a change in the shutter speed requires an equal and
opposite change in the aperture. The AE-1 PROGRAM makes this change in
aperture automatically by means of the shutter-speed priority AE mode. In
Programmed AE, the camera automatically chooses a combination of shutter speed
and aperture for correct exposure.
Strongly suggest you to Download a copy of the PDF file to see the relationship
between shutter speed, opening of the diaphragm and the shutter curtain traveling time.