What causes red-eye in digital photos and how does image-editing software remove it?
The red-eye in your images is caused by light reflecting off the retina, which is a fine network of nerves in the back of the eye, an area rich in blood. Although the retina is not normally visible, if the flash is on the same plane as the lens, the light bounces directly back into the lens and the retina becomes visible in the photo, causing red-eye. This has become more common with smaller compact digital cameras where the flash is very close to the lens. Have you ever noticed that a professional photographer's flash is mounted high on the camera? Sometimes a pro will even take it off the camera.
The fact that the pupil is expanded to its maximum under most flash conditions only exaggerates the problem. Cameras with red-eye reduction will fire the flash two or more times: the first time to constrict the pupils, the second time to expose the image.
Red-eye-reduction software keys on the red and applies a correction. The correction is usually made by desaturating the color in the area. This will appear more natural than if it were filled with black or matched to the surrounding color of the iris (both treatments tend to make your subject look like an alien).
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What causes red-eye in digital photos?
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