Why People Have Red Eyes in Flash Photographs?

We all have seen photographs in which people have spooky red eyes. These are photos taken at night with a flash. The question is where the red eyes come from.

The red color comes from the light that reflects from the retinas in the person’s eyes. A special reflective layer is present in the retina of many animals like cats, dogs and deer, called the tapetum lucidum. This layer acts almost like a mirror at the backs of their eyes. If a flashlight or headlight is struck into their eyes at night, their eyes will shine back with bright white light.

Encyclopedia Britannica says the following about tapetum lucidum:

Among most of the nocturnal vertebrates, the white compound guanine is found in the epithelium or retina of the eye. This provides a surface which is like a mirror. The tapetum lucidum acts like a mirror like surface which reflects light outward allowing a second chance for its absorption by visual pigments at very low light intensities. Tapeta lucida produces the familiar eye shine of nocturnal animals.

Red Eye Why People Have Red Eyes in Flash Photographs?

Tapetum lucidum is not present in the retinas of human eyes. If light is shined into the eyes of human at night, there will be no reflection. The camera flash is bright enough, however to cause a reflection from the eyes of retina, the red color is from the blood vessels nourishing the eye.

There is a feature of ‘red eye reduction’ in many cameras. These cameras give flash twice, once right before the picture is taken and then again to actually take the picture. The first flash causes the pupil of the person to contract reducing the ‘red eye’ significantly. There is another trick to turn on all the lights in the room which also contracts the pupil.

Another technique to reduce ‘red eye’ in picture is to move flash away from the lens. Many small cameras have a flash which is only an inch or two away from the lens, this makes the reflection come right back into the lens and shows up on the film. If the flash is detached and held several feet away from the lens, it is very helpful. The flash can also be tried bouncing off the ceiling if that is an option.

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