What Causes the Red Eye Effect?

The red-eye effect appears when flash photography is used at night because the eye is unprepared for the sudden influx of light. Photo courtesy of

Have you ever wondered why your eyes sometimes resemble those of a spooky vampire in photographs? Your modified appearance is caused by the red-eye effect, which occurs most often with flash photography in dark environments. This pesky phenomenon is actually a result of the eye’s biology interacting with the camera’s lighting mechanism.

The human eye can effectively adjust to different light conditions, but this adaptation is also what leads to the red-eye effect. The eye regulates the amount of entering light by contracting or expanding the pupil. At night, your pupils will accordingly enlarge to extract more light from their surroundings. However, this dilation also leaves your eyes unprepared for the sudden burst of light from a camera flash.

When light from the flash enters your eyes under these conditions, your pupils are unable to contract fast enough to prevent the light from reflecting off of the red blood vessels of the choroid, which is a layer of connective tissue in the back of the eye that is responsible for nourishing the retina. As a result, the camera will pick up the reflection, resulting in the red-eye effect. Interestingly, due to their increased “dark adaptation,” children more commonly have red eyes in photographs.

The amount of melanin, a light-absorbing pigment in the eye, also has a role in the red-eye effect. Individuals with lower levels of melanin will absorb less and thus reflect more light, explaining the high incidence of the red-eye effect in albinos.

Though the cause of this effect is wired in the biology of the eye, some cameras can reduce red-eye by sending a few preliminary flashes before the final flash to give the pupils enough time to contract and adapt to the increased-light conditions. Another way to reduce the effect is to avoid looking directly into the camera lens, which will reduce the reflection of light. Finally, if all else fails, modern image editing software, such as Photoshop, can remove the red discoloration.