In some photographs, why do we see the eyes of human beings or animals with glow in the middle –the red-eye phenomenon?
When human beings or even animals are photographed in low light conditions (with a flash), then photographs may show the subjects with red eyes. Of course, the subject has to look straight into the camera. These red eyes or "Dracula eyes" as they are called are a major source of problems for professional photographers, and television and video filmmakers.
Why does this happen?
For this, we have to know a bit about the human eye. The main parts of the human eye are the pupil, lens and retina. The pupil of a human eye expands or contracts to admit more or less light to the lens. The lens focuses the light on the retina. The retina is like a screen. Photo receptors on the retina respond to the images formed and our brain interprets these responses. The retina is gorged with tiny blood vessels that supply blood to the photo receptors.
Ciliary muscles control the size of the pupil. When the light is intense, the ciliary muscles contract and the pupil becomes small. When the source of light is dim, the ciliary muscles relax and the size of the pupil is expanded to admit more light into the eye.
When a subject is being photographed in low light conditions, the pupils of the subject are in an expanded condition. The flash throws intense light for the duration of the photograph on the subject. This happens in a fraction of a second. The time is too short for the dilated pupils to contract back.
The red eye caught in the photographs is the red glow of the blood flowing through the retina. The flash illuminates the reddish retina at the back of the eye.
In modern flash cameras, corrections are built in to avoid these dreaded red eyes.
Some cameras avoid red eye effects by using the pre-flash function of many flashes. Several preliminary flashes are first emitted so that the pupils close down before the actual photo is taken. This tones down the effect. Digital cameras have a separate provision in the software so that the red eyes are eliminated.