The red-eye effect on a photo of a baby.

The "green-eye" effect on a photo of a yellow-eyed cat.

The red-eye effect in photography is the common appearance of red eyes on photographs taken with a photographic flash. The light of the flash occurs too fast for the iris of the eye to close the pupil. The flash light then illuminates the blood-rich retina at the back of the eye, resulting in a 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 gray or blue eyes and in children (whose eyes have larger pupils and less pigmentation than adults).

In many species the tapetum lucidum, a light-reflecting layer behind the retina that 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, or green eyes in flash photographs.

Retinoblastoma is a cancer of the eye that often causes the appearance of a "white eye" effect instead of the expected red eye.

The red eye effect can be prevented in a number of ways. Placing the flash away from the camera is the most preferrable method. This way, the flash light cannot enter the eye directly and the light reflected by the retina will not be captured by the camera's lens, thus the eyes will appear natural. Another method is to take pictures with no flash (either by increasing the ambient lighting, or by digitally post-processing the picture to increase its brightness). Many modern cameras precede the flash with a period of bright light (the "anti red eye system") allowing the iris to close. Professional photographers prefer the former approaches, as the anti red eye system does not always prevent red eyes (for example if people look away during the pre-flash), and in any case people with small pupils do not look natural on photographs. Various graphics editing software packages have functions to automatically remove red eyes from digital photographs.

If photos or videos are shot with infrared-sensitive equipment, the eyes also usually look unnaturally bright. The reason is the same: the blood-rich retina.