Canon AE-1 Manual page 22

Battery Check and Stopped-Down Metering Index Mark
This battery check index mark serves also as the stopped-down metering index
mark for use with Canon FL lenses and other similar manual lenses, when
exposure measurement is performed with a stopped-down diaphragm.
Manual Aperture Control "M" Signal (LED) When the aperture ring is not set at the "A"
mark, you cannot get the correct exposure in AE photography. When the aperture ring is set at
any position other than the "A" mark, the manual aperture control "M" signal will blink as a
warning. Also, when Canon FL lenses, Bellows or the like are used, this warning signal
flashes on and off when exposure measurement is performed.
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 photo is taken at a shutter speed of 1/250 sec.
and above, the movement will be frozen. If, 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.
2) Effects of Changing the Aperture Because this camera is an AE camera with shutter
speed priority, when you change the shutter speed, the aperture will also change. If you
change the speed by one gradation, the aperture also changes the equivalent of one gradation.
Aperture changes have an effect on the photographic expression as follows: The lens aperture
controls the zone of sharpness in the subject field which is observed in the viewfinder or
recorded on the film.
Aperture Priority Photography After having given careful thought to the results of aperture
adjustments, when the f/stop has been determined before shooting, press the exposure preview
switch while looking into the viewfinder. Then turn the shutter speed dial until the meter
needle on the right of the viewfinder reaches the f/stop desired.
Depth-of-Field When a certain subject is brought into focus, there is only a limited range in
the foreground and background of the subject which can be kept clearly in focus. This zone of
sharpness in the subject field is depth-of-field.