I recently made a discovery:
Everyone is familiar with red eye or the red eye effect – those picture-ruining red eyes in photographs taken in dim light with a flash. Red eyes also appear in pictures of animals, although with animals the eyes can sometimes appear yellow or even blue or green. Although curious about the color variations, I naively assumed the same physiology was responsible for the red eyes in human and animal photographs. Wrong!
The red eye effect occurs when a flash is used in dim settings. The retina of the eye is covered with tiny red blood vessels. In low light the iris opens wide and the pupil is large. The flash reflects off of the blood rich retina and shows up in the picture as a red eye. This effect can be eliminated by using a pre-flash or taking several flash pictures in succession. The pupil closes more with each flash resulting in less reflected red blood. A more efficient method is to position the flash away from the plane of the camera so the flash cannot reflect directly back to the camera.
Eyeshine is independent of the red eye effect. Eyeshine occurs in vertebrates, usually noctural or deep sea, that possess a tapetum or tapetum lucidum. Owls, nightjars, kiwis and other night birds exhibit eyeshine as do cats, dogs, horses and cows among many other species. Humans do not have a tapetum. The tapetum is a layer of tissue behind the retina that reflects visible light back through the retina thus increasing the amount of light available to the eye’s photoreceptors. The light reflected from the tapetum causes eyeshine in pictures. The tapetum, constructed differently depending on the species, reflects light by various physiological mechanisms resulting in the different colors displayed by different animals -for example, nocturnal birds, red; horses, blue. The tapetum is also iridescent and as a result the color of eyeshine can also vary according to the angle at which it is seen.
These owls’ red eyes are the result of eyeshine, not the red eye effect as I previously thought. The variation of color due to iridescence is visible in the varying shades of red to yellow in the owls’ eyes. In one great horned owl picture there is a eye displaying eyeshine while the other eye, due to the angle of the flash, looks normal. The barn owls (Tyto alba) were photographed in our barn (Lookout CA) while the great horned owls (Bubo virginianus) live in a barn on Ash Creek Wildlife Area (Modoc County CA).