The eye is a wonder of evolution that has resulted in our abilities to sense the world around us in a way that blind mole rats can only imagine! Light comes in to the eye through the cornea. That light is then perceived by the retina and its images are sent to the brain via the optic nerve. The amount of light that gets to the retina is controlled by the pupil. Should it be a bright sunny day, the pupil constricts and only allows a small amount of light in. On dark nights, the pupil dilates and becomes quite large to allow as much light in as possible. Sitting between the retina and the sclera (the white part of the eye) lays a layer of connective tissue called the Choroid. This layer provides oxygen and nutrients to the outer part of the retina. Compared to the retina, the Choroid is an extremely vascular area containing copious amounts of blood vessels.
When the light from a camera flash goes off, the pupils do not have time to constrict, and so a large amount of light is reflected off of the Fundus (the interior surface of the eye). Due to the large amount of blood in the Choroid, the light picked up by the camera lens appears red. Since the angle the light goes in to the eye is the same angle that it will reflect out, the closer the flash is to the camera lens, the greater chance the reflected light will be seen by the lens. The darker the environment a person is in, and thus the wider their pupils are, the greater the chance of having eyes of the damned.
Other factors contributing to the red-eye effect is the amount of melanin in the layers behind the retina, and the age of the person being photographed. Light skinned people with blue eyes tend to have less melanin in the fundus. This leads to a more pronounced red-eye effect, compared to dark skinned people with brown eyes. The same is true for other animals as well. The role melanin plays can be seen in pictures of animals with heterochromatic eyes (two different colored eyes). Should the animal have one blue and one brown colored eye, the blue eye will show a pronounced red-eye effect while the brown will not.
Children will also tend to get a red-eye effect more readily then do their adult counterparts. This is because a child’s pupil will dilate faster than adults, in low light situations.
There are a few very simple things you can do to prevent the infamous red-eye. One technique is to constrict the pupils just before the picture is taken. For instance, camera’s with red eye reduction settings use a two flash system in which the first flash causes the pupil to constrict, and the second flash is when the picture is actually taken. Should you simply turn on more lights, your pupils will also constrict. Another technique is to move the flash away from the lens. This will increase the angle at which the light enters the eye, and thus decreasing the chance it will reflect back to the camera lens.