The phenomenon known as "red-eye" is caused by light from
a flash reflecting from the inside rear surface of the eye (the
retina) which is covered with tiny blood vessels.
In a dark environment, the pupils of the eye become enlarged so as
to allow in more light. When a picture is taken with flash, a large
part of the retina is exposed and the reflected light level is high
enough to cause the eyes to appear bright red. To counter this effect,
the anti-red eye setting on the camera delays the flash and
illuminates a small bright light on the camera, pointing at the
subject, which will cause the pupils of anyone looking at it to to
reduce in size before the picture is taken
This compensation only works if the subjects stare at this anti-red
eye light and if their pupils react in time.
