The appearance of red eyes on people and animals when pictures are taken with flash. In dimly lit environments, the eye's iris opens to let in more light. The light from the flash unit reflects back from the retina, which is full of blood vessels; hence, the red look.
Some cameras have an option that produces a flash of light just ahead of the actual flash. The first flash causes the iris to close, eliminating some of the reflection.
Automatic and Manual Removal
Some digital cameras have red eye removal, which removes the red before the image is stored in the camera's flash memory. The removal may be automatic or an option that can be enabled and disabled. However, all photo editing programs have red eye removal. It requires that the user highlight the area around the eyes and either select a default removal function or adjust the amount of reduction. Some programs offer options for replacing the red with a color, which is helpful for pictures where the eyes are an important focal point.
The red can always be removed manually in an image editor by carefully replacing the red with the correct eye color. Although this takes a fair amount of practice to do it well, the results are often the best.
Red Eye Removal
In Nabocorp's excellent cam2pc photo editor, red eye removal is accomplished by sliding a lever to the desired result using the adjustment in the dialog box at the bottom left. (Images courtesy of Nabocorp Softwares, www.nabocorp.com)