What causes one pupil to be red in photographs and the other pupil to remain black?

Answer 1

The red-eye effect in Photography is the common appearance of red pupils in color photographs of eyes. It occurs when using a photographic flash very close to the camera lens (as with most compact cameras), in ambient low light. Red-eye effect appears in the eyes of humans and animals that have no tapetum lucidum, hence no eyeshine, and rarely in animals that have a tapetum lucidum. The red-eye effect is a photographic effect, not seen in nature.

Because the light of the flash occurs too fast for the pupil to close, much of the very bright light from the flash passes into the eye through the pupil, reflects off the fundus at the back of the eyeball, and out through the pupil. The camera records this reflected light. The fact that the reflected light is red often is attributed to the ample blood supply of the retina and/or chorium, but this is incorrect. The red-eye effect is due to the color of the fundus, which is due to melanin, a pigment, principally located in the retinal pigment epithelium (RPE).

Although in many animals the RPE is black, in humans it is brown. Red-eye effect is more pronounced in people and animals with lighter colored eyes, especially grey or blue eyes, and in children. In both groups, all tissues of the eye have relatively less melanin. In a pale eye, and in a child's eye, not only does the iris have less melanin to stop stray light from passing through it, but also the retina and choroid have less melanin to absorb stray light that enters the eye. The role of melanin in red-eye effect is most clearly seen in animals with heterochromia: only the blue eye displays the effect. The effect is still more pronounced in humans and animals with albinism. All forms of albinism involve abnormal production and/or deposition of melanin. Red-eye effect is seen in photographs of children also because children's eyes have more rapid dark adaption: in low light a child's pupils enlarge sooner, and an enlarged pupil accentuates the red-eye effect. More information on the related link below.