Why Do Eyes Sometimes Turn Red in Photographs?

The red eyes in some flash photographs are not created by the camera. They are a view into the blood rich interior of the eye.

In dim surroundings, the pupil opens wider so that more light can reach the retina. If a camera flash is positioned close to the lens, its light enters through that enlarged pupil and strikes the back of the eye.

Some of the light is absorbed by the retina and the darker tissue beneath it. Some is reflected back toward the camera. Blood vessels and pigments inside the eye give much of that returning light a red or orange appearance.

The effect is strongest when the flash, pupil, and camera lens are nearly aligned. The camera is then positioned to collect light returning along almost the same path by which it entered. It also explains why the effect is common with compact cameras and phones using a flash close to the lens.

Red eye reduction settings often fire a preliminary light or flash. This causes the pupils to narrow before the photograph is taken, reducing the amount of light entering and returning from the eye. Moving the flash farther from the lens changes the reflection angle and can also prevent it.

Animals may show green, yellow, blue, or white eye shine instead. Many nocturnal animals possess a reflective layer called the tapetum lucidum, which sends light through the retina again to improve vision in darkness. Humans do not possess this layer.

A red eye photograph is therefore not merely a digital mistake. For an instant, the camera flash has illuminated the inside of the eye and photographed the light coming back out.