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The red-eye effect in photography is the common appearance of red pupils in color photographs of eyes. It occurs when using a photographic flash very close to the camera lens (as with most compact cameras), in ambient low light. The effect appears in the eyes of humans, and of animals that have tapetum lucidum.
Theatrical followspot operators, positioned nearly coincidentally with a very bright light and somewhat distant from the actors, occasionally witness red-eye in actors on stage. The effect is not visible to the rest of the audience because it is reliant on the very small angle between the followspot operator and the light.
Because the light of the flash occurs too fast for the pupil to close, much of the very bright light from the flash passes into the eye through the pupil, reflects off the fundus at the back of the eyeball and out through the pupil. The camera records this reflected light. The main cause of the red color is the ample amount of blood in the choroid which nourishes the back of the eye and is located behind the retina. The blood in the retinal circulation is far less than in the choroid, and plays virtually no role. The eye contains several photostable pigments that all absorb in the short wavelength region, and hence contribute somewhat to the red eye effect. The lens cuts off deep blue and violet light, below 430 nm (depending on age), and macular pigment absorbs between 400 and 500 nm, but this pigment is located exclusively in the tiny fovea. Melanin, located in the retinal pigment epithelium (RPE) and the choroid, shows a gradually increasing absorption towards the short wavelengths. But blood is the main determinant of the red color, because it is completely transparent at long wavelengths and abruptly starts absorbing at 600 nm. The amount of red light emerging from the pupil depends on the amount of melanin in the layers behind the retina. This amount varies strongly between individuals. Light skinned people with blue eyes have relatively low melanin in the fundus and thus show a much stronger red-eye effect than dark skinned people with brown eyes. The same holds for animals. The color of the iris itself is of virtually no importance for the red-eye effect. This is obvious because the red-eye effect is most apparent when photographing dark adapted subjects, hence with fully dilated pupils. Photographs taken with infra-red light through night vision devices always show very bright pupils because, in the dark, the pupils are fully dilated and the infra-red light is not absorbed by any ocular pigment.
The role of melanin in red-eye effect is nicely demonstrated in animals with heterochromia: only the blue eye displays the effect. The effect is still more pronounced in humans and animals with albinism. All forms of albinism involve abnormal production and/or deposition of melanin.
Red-eye effect is seen in photographs of children also because children's eyes have more rapid dark adaption: in low light a child's pupils enlarge sooner, and an enlarged pupil accentuates the red-eye effect.
Similar effects, some related to red-eye effect, are of several kinds:
Canon EOS 100 camera with bright light to contract pupils
Motorola Milestone XT720 camera while capturing image with red-eye reduction and xenon flash.
The red-eye effect can be prevented in a number of ways.
If direct flash must be used, a good rule of thumb is to separate the flash from the lens by 1/20 of the distance of the camera to the subject. For example, if the subject is 2 meters (6 feet) away, the flash head should be at least 10 cm (4 inches) away from the lens.
Professional photographers prefer to use ambient light or indirect flash, as the red-eye reduction system does not always prevent red eyes — for example, if people look away during the pre-flash. In addition, people do not look natural with small pupils, and direct lighting from close to the camera lens is considered to produce unflattering photographs.
Red-eye removal is built into many popular consumer graphics editing software packages, or is supported through red-eye reduction plug-ins; examples include Adobe Photoshop, Apple iPhoto, Corel Photo-Paint, GIMP, Google Picasa, Paint.NET and Microsoft Windows Live Photo Gallery. Some can automatically find eyes in the image and perform color correction, and can apply it to many photos at once. Others may require the operator to manually select the regions of the pupils to which correction is to be applied. When performed manually, correction may consist of simply converting the red area of pupils to grayscale (desaturating), leaving surface reflections and highlights intact.
In a photograph of a child's face, red-eye in one eye but not the other may actually be leukocoria, which may be caused by the cancer retinoblastoma, and the child's eyes should be examined by a general physician. 
The feature film Blade Runner intentionally induced a red-eye effect to indicate that certain characters were "replicants," an artificial life-form in the story. The effect relied on a semi-silvered mirror directing light at the actors on the same incidence as the camera.
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