In photos taken at night with a flash, people often appear as zombies with their spooky red eyes. Do you know where the red eyes come from in color photographs? Well, it’s attributable to the light of the flash which occurs too fast for the pupil to close and thus, allowing the light to reflect off of the retinas in our eyes. The bright flash may be too much for the eye to make any possible adjustments to negate red-eye effect.
Normally, you can’t notice any reflection when you shine a flashlight in a person’s eyes at night. The camera flash, however, is too powerful for the retina to absorb fully. When our retina reflects off this light, our eyes look red because of the presence of blood vessels. The camera records this reflected light.
However, the amount of red light emerging from the pupil is directly proportional to the amount of melanin in the layers behind the retina. While light-skinned people with blue eyes show a much stronger red-eye effect, it’s weaker in dark skinned people with brown eyes. Moreover, as you would have noticed, the red-eye effect is more noticeable in photographs of children. It is so because children’s eyes have more rapid dark adaption.