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Chapter 4. How to Use Your Flash > What the Heck Causes Red-Eye, and How Do I S...

What the Heck Causes Red-Eye, and How Do I Stop It?

We've all seen it, we all hate it—red-eye is the scourge of photography. But the glowing red eyeball effect that is the hallmark of so many bad photos is caused by a cool bit of biology.

When light is low, the eye's pupil opens up to let in more light. This low-light condition is the same one that causes your camera to set off a flash. Light from the flash travels in a straight line to the eyeball, where it enters the wide-open pupil, hits the back of the eyeball, and reflects off all the neat-but-yucky stuff back there. The light, now tinted by the red blood in the vessels of the eyeball (I told you this was cool) travels back to the camera, where the redness is captured.

There are two ways to overcome red-eye. The first is to use a flash that's not so close to the lens. As the distance between the lens and the flash gets greater, the angle of the light entering the eyeball changes, and that causes the light from the back of the eye to reflect away from the camera.

The second method doesn't work as well, but it's the only solution for a camera that can't use an accessory strobe. Operating in red-eye reduction mode (see “Flash Modes: More Than Meets the (Red) Eye,” below), the camera flashes multiple times very quickly and then emits one longer flash. The several quick bursts of light make your subject's pupils contract, which allows less light to enter or exit, and reduces the amount of red-eye.

The key to flash photography overall is being smarter than your camera. Let the PhotoCoach show you the tricks to knowing when to turn the flash off, when to turn it on, and how to modify the light in order to take better pictures than ever.

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