The Science Behind the Red-Eye Effect

A frustrating problem for many beginning photographers is the “red-eye effect.” Everyone has seen the photos with the demonic, crimson eyes. Most softwares now make red-eye removal fairly easy – the computer simply looks for red pixels in the suspect area and substitutes black ones. This solution works surprisingly well, but it can sometimes leave the eyes looking dull and lifeless. Any catch lights in the eye can get thrown out with the red pixels. Ideally, we’d like to understand the causes of red-eye and avoid them during the capture.

Red-eye photos are, in effect, photographs of the back of your subject’s eyeball. As shown in the diagram below, an intense flash enters the pupil from a direct angle, reflects off the blood vessels in the back of the eye, and exits the pupil headed straight back to the camera. Why is the exiting light red? The light picks up most of its red color from blood in the choroid, a vascular part of the eye which supplies blood to the retina. Therefore, any photo with red eyes is literally blood stained. Some color filtering is also completed by the lens, which blocks blue wavelengths below 400nm.

The Science of the Red-Eye Effect - Light enters the pupil and bounces off the back wall of the eye, picking up its characteristic red color along the way.

If we study the diagram carefully, we can devise a number of strategies against red-eye. As shown, the red effect occurs when a straight-on flash impinges the eye and reflects directly back. Therefore, one avoidance strategy is to separate the flash from the optical axis by using an off-camera flash. You can also aim your flash at the ceiling or a reflective wall if your flash is so capable. If you are using a pop-up flash or a point-and-shoot camera, however, the situation is more challenging. Here is a list of options for combatting red-eye (in order of preference):

Use an off-camera flash or use a reflective off-axis surface to direct your light.

Increase your camera’s ISO setting – this will increase the sensitivity of your camera’s sensor and may allow you to photograph without the flash.

Increase the ambient lighting – this will both lessen your need for a flash and it will allow your subject’s pupils to contract. Smaller pupils mean less light coming into the pupil and less light leaving.

Use your camera’s red-eye reduction option – this will fire a series of pre-flashes which will trigger the pupil to contract.

Have your subject look away from the camera. Use the opportunity to take a creative profile photo instead of another boring straight-on shot.

Eyeshine

A similar effect which happens with animals is called eyeshine. Instead of red backscatter, this effect causes an equally haunting iridescent glow in the eyes. Take a normally cute and cuddly creature and photograph them with a harsh flash and they will look absolutely demonic. It is also the effect behind the “deer in the headlights” look. The image below illustrates the concept of eyeshine.

The Science of Eyeshine - Light reflects off the tapetum lucidum, picks up its iridescent color and reflects back through the pupil.

The key to eyeshine in some animals is the addition of a reflective layer behind the retina. This layer is called the tapetum lucidum (Latin for “bright tapestry”). The obvious advantage of this design is the increased availability of light for the retina. Humans have recently used this idea for a variety of inventions -including reflective pavement markers and low-light sensitive camera sensors.

Eyeshine in a female Northern Cardinal

Eyeshine can prove to be even more confounding than red-eye when photographing animals. Onboard flashes may simply be impossible to use in some cases, meaning that you must increase the ambient light, use an off-camera flash, or simply be happy with a profile shot of Fido.