Last night someone asked me what causes red-eye during flash photographs. My simple explanation at the time was that the light from the flash was reflecting off the back of the retina. Red-eye is more prevalent at night when the eye’s iris is widest open to let in the most light. Some camera manufacturers try to overcome this with a special red-eye-reduction flash; those annoying multi-flash, red-eye-reducing cameras attempt to fool the eye into thinking that there’s an abundance of light and to subsequently close the iris.
Naturally, I consulted my medical references when I got home. I was pretty darn near close, although it makes me feel a bit like Cliff Claven when I get these things even slightly wrong.
No, it’s not technically the iris that opens and closes. The iris is the colored part of the eye, the muscles that surround the pupil, and is technically one of the body’s many sphincter muscles. Its purpose is to open and close the pupil, the hole in the center of the eye that lets in the light.
Yes, I said “sphincters”. Go ahead and get over your giggling so you can go back and re-read that and maybe learn something.
“My, what a nice pair of sphincters you have!” — isn’t exactly an opening line you’re likely to hear often. Stick with “My, what pretty red eyes you have!” You’ll get much farther with girls like Cheska.
You did notice her red eyes, didn’t you? Or were you unable to focus your eyes on anything other than her round objects?
So, why red? I couldn’t remember at the time, but if you remember what’s back there in the retina you can easily figure it out: rods, cones, and blood vessels. The light is reflecting off the blood vessels in the retina right back to the camera. And since everyone I know has red blood, well, you “get the picture”…
The simplest methods to prevent red-eye? Use an off-camera flash. When the flash is too close to the lens, as on most point-and-shoot cameras, you get that lovely red demon-like reflection.
Second best? A little touch-up with Adobe Photoshop, iPhoto, GIMP, or Pixelmator. Doesn’t actually prevent red-eye, but any of the major image-editing software packages can easily remove the redness, or have readily available free plugins to do the job near automatically.
But What About Dogs and Cats?
Thanks to ‘Sir Matthew of Wembly‘ for pointing out that while red eye in humans is caused by reflections from red blood vessels at the back of the eye, other animals reflect colors other than red. The explanation may require a higher level of vocabulary than Matthew is accustomed to.
The epithelium or retina in many animals (including dogs, cats, and deer) have a special reflective layer called the tapetum lucidum. It acts almost like a mirror at the back of their eyes and enables the animal to see in dimmer light than it would otherwise be able to.
Consisting of a compound called guanine, the tapetum lucidum reflects light outward and thereby allows a second chance for its absorption by visual pigments at very low light intensities. The visual pigments present in the eyes of these animals produce the non-red eyeshines often seen from nocturnal animals.
Humans, of course, do not have a tapetum lucidum layer in their retinas — therefore, the reflection off the retina shows the red color from blood vessels. As I clearly stated above.
Interestingly, dogs and other animals that have complete heterochromia iridum (or bi-colored eyes) usually give off flash reflections of different colors, too. Humans with heterochromia (such us Kate Bosworth, Christopher Walken, and Mila Kunis) still reflect blood red in both eyes.