Taking flash pictures of someone often gives their eyes a scary-looking red glow. You’ll find out why in this Science Update.
Getting the red out. I'm Bob Hirshon and this is Science Update.
Today's Why Is It question comes from Breana Downey of Carlisle, Pennsylvania. She wants to know why sometimes people's eyes glow red in photographs.
We asked David Williams, director of the University of Rochester's Center for Visual Science. He says red-eye occurs when your camera's flash reflects off the back of the eye into the camera lens.
The reason it looks red is because there's a rich bed of blood vessels that line the inside of the eye and supply the retina. So it's that blood which is what you're really looking at.
He says you can avoid red-eye by using a camera where the flash and the lens are farther apart. That way, the light will reflect off at an angle, and not straight back. If you have a science question, we'll answer it in a flash. Call us at 1-800-WHY-ISIT. I'm Bob Hirshon for AAAS, the Science Society.
Making Sense of the Research
This question reminds us that the pupil, even though it looks like a black dot, is actually an opening. It doesn’t open into a hollow chamber, but to the translucent jelly that fills the eye. Pupils allow light to pass through this jelly and onto the back of the eye, or the retina. The retina receives the light and sends nerve signals to your brain to interpret it. That’s how you see.
A camera works in a similar way. Instead of a pupil, it has a shutter that allows light to pass through the lens and onto a piece of film. The film records the image just like your retina does.
Back to the eye: Normally, when you look at someone, some of the light passing through his or her eyes bounces around inside the eyes and reflects back to you. However, the amount of this light is so small that the pupils just look black. A camera flash, however, sends a tremendous amount of light into the eye in a very short time. If the camera lens is close enough, some of this light ricochets back off the retina and into the lens. And because the back of the retina is filled with blood vessels, the pupils look red.
Photographers have been tearing their hair out over this problem for over a century. As you heard, one way to deal with it is to back off from the person you’re photographing: the farther away the camera lens, the smaller the amount of light that will reflect back into it. Some cameras also include a red-eye reducing dimmer flash. This is a small flash that goes off a few seconds before the main flash. The purpose of this flash is to cause the subject’s pupil to constrict (just as it does when you go out in the sun), which makes it harder for light to reflect back at the camera.
What’s bad for photographers has been great for eye doctors, however. Back in the 19th century, the German physician Hermann von Helmholtz discovered that he could look inside the retina by holding a bright light near his own eye and shining it directly into a patient’s pupil. This allowed doctors to see the inside of the eye non-invasively, and ophthalmologists still rely on this simple technique today. Now try and answer these questions:
- Why do camera flashes cause red eyes?
- The red-eye effect is worse in people with blue or pale eyes. Why do you think this is?
- Why doesn’t the red-eye effect happen in bright sunshine?
- Can you think of another strategy that would reduce the red-eye effect?
The Access Excellence article How We See: The First Steps of Human Vision features extensive information on sight. Diagrams and illustrations accompany the text.
How Cameras Work, by How Stuff Works, explains the inner workings of cameras in great detail.