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Grab a partner and look into his or her eye – or stare deeply into it, if appropriate – in a normally lit room. The quick and dirty version of how you’re able to view each other is this: Light enters the eye through the cornea, the clear outer dome, and goes through the pupil. Then it travels to the cornea, which focuses it on the lens. The lens further focuses the light and spreads it across the retina. The retina receives the light and transmits signals via the optic nerve to the brain, which interprets the image.
As light enters the eye, some of gets reflected back, but the amount of light in most situations is so small, you wouldn’t even know it. Right now, your partner’s pupils look black and everything’s normal. Now grab a camera and take a picture of your partner with the flash on. There’s that demonic red eye.
Here’s what happened: When you took the picture, the camera flash sent a lot of light into the eye in a very short time, the light reflected off the back of the eye and out through the pupil and, because the camera lens is close to the flash and able to capture images very quickly, it caught the light reflecting back out.
Seeing Red
So why is that light red? Because the fundus, the interior surface of the eye that includes the retina, is loaded with melanin, a pigment that gives it a brownish-reddish color. Was that anti-climactic? Sorry.
Red eye is fairly easy to curb by using the “red eye reduction” setting found on most digital camera flashes. This setting causes the flash to go off once before the picture is taken, which causes the subject’s pupils to contract and let less light in and out, and then another time to take the picture. Cameras with a flash farther away from the lens also reduce red eye because the flash hits the subject at a different angle than lens captures it.
Of course, red eye isn’t all bad. The same mechanics of light reflection that ruin photos also allow doctors a non-invasive way to see inside the eye. Hermann von Helmholtz, a German physician, discovered in the 19th century that he could examine the retina by holding a bright light near his eye and shining it into patients’ pupils.
If you’ve got a burning question that you’d like to see answered here, shoot me an email at flossymatt (at) gmail.com. Twitter users can also make nice with me and ask me questions there. Be sure to give me your name and location (and a link, if you want) so I can give you a little shout out.
Comments (12)
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Now you have to answer the question of why the light reflects back green (or yellow) in cats.
posted by zantimisfit on 10-24-2008 at 2:01 pm
zantimisfit:
easy…cats have green (or yellow) blood
problem solved
posted by chris on 10-24-2008 at 2:12 pm
I read a story about blogging moms from different countries the other day. They were exchanging pics of the babies when the other mom was alarmed by a white mass on the baby’s eye. She called up the woman and told her it could be an awful cancer. Turns out she was right and saved that baby’s life thanks to the flash of the camera. Little bit still had to have the eye removed though. It’s better than death, I would assume.
posted by karissa on 10-24-2008 at 2:18 pm
@zantimisfit
That would be the tapetum lucidum, a layer of tissue behind or in the retina of certain animals, like cats, that helps them see in low light and darkness. Light reflecting back from the eyes of an animal with tapeta lucida is called “eyeshine” and occurs in a range of colors (why that is is beyond me). Cats’ eyes just happen to shine green and yellow, but rodents’ shine red and other animals shine different colors.
posted by Matt on 10-24-2008 at 2:30 pm
There was a doctor on the CBS Early Show just this week who said that if in a group picture, your child is the only one who DOESN’T get red-eye, to immediately take them to the doctor because it signified a problem. They didn’t specify what that problem was.
posted by Vickey on 10-24-2008 at 3:00 pm
That’s interesting. My cat normally has golden/green eyes. I have several different pictures of her. In one of them, she has red eyes, but in another she has blue eyes. Very bright blue.
posted by Tara on 10-24-2008 at 4:26 pm
Light enters the eye through the pupil in the centre of the iris. The cornea refracts light.
posted by AMR on 10-24-2008 at 4:35 pm
Random, my white friends tend to get red eye on photos more often than my african friends. Can any1 explain why this is?
posted by B on 10-24-2008 at 11:01 pm
A better solution to red eye if you are into photography is to have the flash either “bounce” off a ceiling or wall, or just separate the flash unit from the camera and attach it with a wire. It is the line of fire of the flash being lined up so perfectly with the line of shooting of the lens that catches the “red eye” reflection. Just get the flash off the camera or bounce it. Yes it takes more than your average point and shoot to do this.
With a point and shoot you are probably better to turn the flash off, the ASO speed as high as you can, and hold steady. Use available light.
The “red eye reduction” approach,where the camera fires a series of rapid fire flashes before you shoot, is an attempt to constrict the pupil of the one being photographed and lessen the red eye. It is a sure fire way to not get spontaneous informal portraits. It alerts your subject and whatever photo moment you had seen is gone.
Lastly , it is easy to fix red eye post production with most photo software, replacing the red with a dark tone.
posted by Charles on 10-25-2008 at 7:43 am
In all my family’s photos only the ones with blue or green eyes get red-eyed. Brown-eyed people like and my father don’t.
posted by MetFanMac on 10-25-2008 at 3:34 pm
Adding to what Charles said above, “red eye reduction” in cameras does just that – it reduces, but does not eliminate red eye. It simply makes it smaller, you’ll still have to REMOVE it later.
Also, the pre-flash not only delays the moment the actual photo is taken, it also chews up precious battery power.
Turn red-eye reduction OFF and fix red eye later on your computer.
posted by Russ on 10-26-2008 at 9:20 pm
Actually, the answer is not quite right. The red light reflex comes from the blood-rich choroid, a layer behind the retina. The retina is fairly transparent, so we are seeing blood vessels. In fact, the more melanin in the choroid, the LESS red the reflex.
posted by Michael on 10-27-2008 at 10:15 pm