What Causes The Red Eye In Photos?

When light enters a person's eye, it's focused on the lens by the cornea, which then travels to the retina before being turned into electric signals for the brain to interpret the image. But, through this whole process, some of that incoming light is reflected back out, as Mental Floss explains. However, so little light

When light enters a person's eye, it's focused on the lens by the cornea, which then travels to the retina before being turned into electric signals for the brain to interpret the image. But, through this whole process, some of that incoming light is reflected back out, as Mental Floss explains. However, so little light makes it back out that you don't notice the reflection. But when you take a flash photo, particularly in low-light conditions, several things happen in a fraction of a second. 

As Defiance Optometric Group explains, a flood of light enters the eye, and the pupil undergoes its natural constriction process. However, that takes time, and before that happens, the light will have already been reflected out, and the camera lens will have already snapped. More often than not, the result is a terrifying reddish glow in the eyes — red because the surfaces of the eye that reflect light back out contain a reddish-brown pigment, and so it looks like red light (per Mental Floss).

Some modern cameras come with a means of mitigating the red-eye effect somewhat. They accomplish this by flashing twice in a brief period of time before taking the picture; the first flash causes the pupils to dilate, while the second provides the lighting the photographer wanted — hopefully with the subject's pupils constricted enough that no light will escape back to the camera.

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