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Treebeard's Stumper Answer
20 October 2000

Red-Eye Blues

I'm still sorting out the 380+ photos I took on our Washington D.C. trip. I'll get them on the Web soon. In the meantime, it's fun looking at the kids' trip pictures at school. One familiar problem in our photos taken at night with a flash is that some people have spooky-looking red eyes. What causes red-eye in our photographs, and how can we avoid it? Why do our eyes look red and not some other color, and why do we only notice this in flash photos? Is red-eye the same as the "eye shine" that helps many nocturnal animals see at night?

A classic red-eye flash photo. The bright highlights are a reflection of the camera flash, but where does the red come from? (I had to guarantee anonymity to use this picture!)
Two Dunn Middle School students with Dorothy's ruby slippers from the Wizard of Oz at the Smithsonian Museum of American History on our recent Washington D.C. school trip. The girls' eyes match the shoes (if you look close), but the flash reflection on the glass is white.

(D'oh, don't point the flash straight at the glass!)

This un-retouched flash photo of Lauren's cat is the ultimate red-eye stumper. Ok, Graybear, explain this!

I took this photo on a hard hike with my dog Mojo
down the shady north-facing Los Laureles Canyon
below my house. I turned off my flash hoping that
a long exposure would pick up the background, but
all I got was the eye shine from Mojo's eyes.
I fiddled with the contrast and gamma settings in my
graphics program, and now you can see that Mojo is
swimming in a rock-lined pool along the creek. The eye
shine is a reflection of the ambient light in the shady
canyon. Is red-eye in photos the same phenomenon?

Cats and many other nocturnal animals have a mirror-like layer at the back of their eyes that reflects background light like tiny searchlights. Humans don't have this tapetum lucidum layer, so red-eye photos are something different. The red color is a reflection of the many blood vessels that nourish our eyes, like the pink color of our lips. We only notice this in flash photos because the camera flash is bright for such a short time, 1/1000th of a second or less, so our pupils don't have time to close down or constrict. One solution is to turn on a light.


Graybear emailed a comprehensive answer to this stumper, including an answer to my direct challenge about the cat's eyes:

Red-eye occurs when the eyes are accustomed to dim light, i.e. the pupils are dilated and the blood vessels in the retina are fully expanded to let in as much light as possible. The light from the flash is then able to illuminate the retina thus allowing the camera to capture the red of the blood vessels.

Ways to prevent red-eye:

  1. Add as much other light as possible so that the eyes of the subject are not as dilated.
  2. Diffuse the light from the flash with a tissue.
  3. Increase the distance between the flash and the camera lens. (Practically impossible with compact cameras.)

Some compact cameras flash several times to attempt to condition the eye before the actual photograph is taken, but they have limited success. Once red-eye has occurred, you can 'correct' it by coloring in the eyes with a special pen or, in the modern era of digital photography, by using software.

As to the picture of the cat, I believe the photo was taken with a compact camera with the flash on the left side of the camera (as you are looking through it). This, combined with the proximity of the subject, caused red-eye in one eye but not the other. Notice, too, how the 'white' of the eye that turned red is a much brighter white than the green eye showing the illumination of the eyeball through the dilated pupil.

My guess is that the cat's blue eye (photo left, cat right) is stuck open, so it can't close in response to the flash and room lights. In the picture, the red pupil looks bit larger than the green one, and the iris is not as well defined. I asked the cat's owners, Chris and Lauren, about this and got this answer:

The cat's name is "Bug". He's an interesting guy. We were told that this breed often has one blue eye, and that deafness in the corresponding ear is usually also present. Bug, however, is totally deaf (maybe that's why his previous owner dumped him at the Humane Society). Being deaf for this cat translates into a very exaggerated "startle" response, so if you don't ease yourself into his peripheral vision, or thump on the floor when approaching him, he becomes a snarling vertical projectile.

Lauren has noticed that the line between his pupil and iris on the blue eye is rather fuzzy, compared to the other eye, and that the pupil is also larger. So, maybe his eye is stuck, as you say. Next time I take him to the vet, I'll have her take a look and offer an opinion also.

Thanks again for the photo, Chris, it's a great one! I would like to spend some time with Bug in a dark room with a flashlight to settle this.

Geoff (who designs optical sensors) also sent in a good answer to the stumper, including an article from the Encyclopedia Britannica and a comment:

I'm surprised that the answer in this article doesn't include an optical description of what's going on, namely that the lens of the subject's eye is recollimating the light that is reflecting (and diverging) from the retina. Therein lie two answers to reducing red-eye: Bounce-flash or diffuse flash, either of which remove the highly directional nature of incident light. Point & shoot cameras use a pre-flash to close down the subject's pupils to reduce the red-eye signature. The usefulness of this effect is another topic.

Camera lenses collect light from an object and focus it to an image, but they can also work in the opposite direction to take light from a source and collimate it into a parallel beam. My "ordinary language" description of the effect is that the nocturnal animals' eyes act like tiny searchlights. This is why animal eyes look so intense at night. They not only reflect ambient light back out through the light-sensitive retina toward the object, they also collimate it into a beam to intensify the reflection back again. These animals are recycling light in an efficient way!

I wonder if there are special adaptations for this backwards reflection? I usually focus on a particular object in space, but the ambient light reflected from the tapetum lucidum comes from all angles. The classic lens optics I learned isn't so helpful with organic lenses that can change shape on the fly. (I know about that first hand. After being near-sighted all my life, I now have increasing trouble reading. Bifocals help, but I really have trouble reading book titles at an in-between distance on the shelf!)

Why do different animals' eyes appear different colors at night? One source on the Quirks & Quarks radio show from Canada (see links below) explains that the perceived color is due to the exact configuration of the riboflavin (or guanine?) crystals that make up the tapetum lucidum, and that the reflection shifts the frequency of the reflected light towards the yellow-green, where animal eyes are most sensitive. Is this frequency-shifting to the optimal color the perceived eye-shine color? How does this work? It's interesting to think that nocturnal animals are actually "seeing" a false color, image-intensified mapping of what they look at!

"Tapetum lucidum" literally means bright carpet, a nice phrase. A Web search on that phrase turns up an abundance of interesting info. Here are some different animal species (and links) that I've found listed as possessing a tapetum lucidum for enhanced night vision. It's interesting how this adaptation appears in such different animal groups!

The Eagle Bluff Environmental Learning Center in Lanesboro, Minnesota offers this nice table of animals you might recognize by their eyes on a night hike. There's lots here for a naturalist to learn!

Iris Color Animal Eyeshine Color Relative Strength

YellowScreech Owlredweak
YellowGreat Horned Owlredmedium
YellowLong Eared Owlslightly redstrong
YellowSnowy Owlslightly redmedium
BrownBarred Owlredstrong
BrownBarn Owlredweak
VariousWhite-tailed Deersilver-whitestrong

Animal eye-shine is an important clue for hunters as well as naturalists. A site about the endangered California Kit Fox has this comment by Dr. Ted Murphy (with tongue in cheek?):

Poachers, when "jacklighting," illegally hunting at night with powerful spotlights, usually detect the lair by the brilliant eyeshine: at a distance that even the animal is not visible in the light, the eyeshine will be. Incidentally, a few humans have this feature and it's often called "beer can eyes" since the reflection resembles the reflection of a beverage can on the side of the road. Such individuals seldom wander in the woods after dark, for obvious reasons.

Here are some Web links for further research:

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