Canon AE-1 Manual page 37

A second LED at the bottom of the aperture scale flashes on and off to warn of
underexposure. Both LED's flash at 4 Hz.
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When you push the battery-test button, the needle should move to the battery-test mark
or below it. The battery-test mark also provides the stopped-down-metering mark
when you're using the FL lenses.
•
The front surface of the maximum-aperture correction pin should be 5.7mm (+0, -
0.2mm) from the front surface of the lens-mounting ring. To make the adjustment, first
loosen the set screw in the center of the maximum-aperture correction pin. You can
then turn the maximum-aperture correction pin clockwise to increase the distance. To
decrease the distance, turn the maximum-aperture correction pin counterclockwise and
turn in the setscrew to hold the adjustment.
•
The flange-focal distance to 41.9mm between the front of the lens-mounting ring and
the film-guide rails or to 42.14mm between the lens mounting ring and the pressure-
plate rails.