Red-eye effect



The red-eye effect in photography is the common appearance of red eyes on photographs taken with a photographic flash when the flash is too close to the lens (as with most compact cameras).

The light of the flash occurs too fast for the iris of the eye to close the pupil. Light is focused onto the blood-rich retina at the back of the eye and the image of the illuminated retina is transmitted to the camera resulting in the red appearance of the eye on the photo. (This principle is used in the ophthalmoscope, a device designed to examine the retina.)

The effect is generally more pronounced in people with grey or blue eyes and in children. This is because pale irises have less melanin in them and so allow more light to pass through to the retina. Children, despite superficial appearances, do not have larger pupils but their pupils are more reactive to light and are able to open to the fullest extent in low light conditions.

In many species the tapetum lucidum, a light-reflecting layer behind the retina which improves night vision, intensifies this effect. This leads to variations in the colour of the reflected light from species to species. Cats, for example, display blue, yellow, pink, or green eyes in flash photographs.