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The light of the flash on most cameras is usually located only a few inches away from the camera lens. When the flash goes off, light goes straight into the pupil and gets reflected off the back of the eye, and straight in to the camera lens. The effect is magnified with dilated pupils–that’s why “red eye” mode of cameras involve a pre-flash before the main flash: so pupils become smaller before the real picture is taken and the red eyes are minimized.
As flash light enters the eye, it is reflected back into the camera lens. The retina (nerve tissue in the back of the eye) is essentially transparent, but the underlying choroid has pigment and a lot of blood vessels. It’s the choroidal blood that gives the reflected light coming out of the eye its reddish hue.
Other ways to minimize red eye
Besides using the red eye mode of your camera’s flash. Reducing red eye can be accomplished by using physics. If red eye is caused by the flash light being sent in a straight line into the eye and then directly reflected back out, it can be decreased by having the flash source away from the camera lens. Angled light going into the pupil will be reflected out at an angle, so less of it will hit your camera’s lens. Consider using a detached flash unit located several feet away from the subject you are shooting, or reflecting the flash off the ceiling.
Does anyone else have any tips for eliminating red eye in photographs?
(Photo credit: freedigitalphotos.net)