Red-eye effect

Missing image Redeye.jpg The red-eye effect on a photo of a baby.

Missing image Cat-greeneye.jpg 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 is focussed by the lens of the eye onto the blood-rich retina at the back of the eye and the image of the illuminated retina is again focussed by the lens of the eye back to the camera 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 grey 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, pink, 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 use of certain medical and recreational drugs cause the iris to dilate and red eye reduction techniques will be less effective when photographing people using such drugs. The alkaloid belladonna was so named because a large iris was supposed to increase the beauty of the human face.

The red-eye effect can be prevented in a number of ways.

1. Using bounce flash in which the flash head is aimed at a nearby pale coloured surface such as a ceiling or wall or at a specialist photographic reflector. This both changes the direction of the flash and ensures that only diffused flash light enters the eye.
2. Placing the flash away from the camera's optical axis ensures that the light from the flash hits the eye at an oblique angle. The light enters the eye in a direction away from the optical axis of the camera and is refocused by the eye lens back along the same axis. Because of this the retina will not be visible to the camera and the eyes will appear natural.
3. Taking pictures without flash by increasing the ambient lighting, opening the lens aperture, using a faster film or detector, or reducing the shutter speed.
4. Digitally post-processing the image to increase its brightness.
5. Pushing the film development to increase the apparent film speed
6. Utilising red-eye reduction capabilities that are built in to many modern cameras. These precede the flash with a series of short low power flashes triggering the iris to contract.
7. Get your subject not to look straight at the camera, instead look at the shoulder of the photographer, by changing the angle red eye will be reduced. Red eye is caused by the light from the flash reflecting of the retina and back to the camera lens, by altering the angle red eye is reduced
8. Increase the lighting in the room so that the subjects pupils are more closed, this is how the red eye reduction works by firing a burst of pulses to close the iris of the eye, a brighter room also has the effect

Professional photographers prefer the former approaches, as the red-eye reduction 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.de:Rote-Augen-Effekt pl:Efekt czerwonych oczu