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Interactive video lesson plan for: What Causes Red Eye In Photos?

Activity overview:

How can a camera's flash make your eyes glow red? Tune in to learn how it works -- and how to prevent it.

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Hi, I’m Jonathan, and I’m here to talk to you about why people’s eyes sometimes appear red in photos. Horrible, glowing red. The glow of eyes that have peered into the abyss and through which the abyss peers back.

Nah, just kidding. It’s simple reflection.

Everything that you can see is reflecting some amount of light. You can see my shirt because it’s reflecting blue wavelengths of light and absorbing the other wavelengths. Black things, like my lav mic and my pupils, absorb most of the light that hits them.

Most pupils look black because they’re shadowy windows to the retina. The retina is lined with the dark pigment melanin to promote light absorption. That gives all the photosensitive cells in the retina the best chance at catching the light coming at them.

The retina contains a lot of those photosensitive cells – some 107 million of them - plus nerves to carry messages from those cells back to the brain. All that stuff needs blood to function, so the retina is also dense with blood vessels.

Red eye is just a glimpse at those blood vessels. You see, camera flashes illuminate everything within their reach – including the blood vessels in the retina. A camera with a built-in flash will have that flash pointed directly at the subject. At the speed of light, the flash bounces off the subject and back to the lens. If the angle is just right, you wind up looking like Hellspawn.

Part of the problem is that if you’re using a flash, you’re in dim light. Meaning that your subject’s irises will be dilated, with lots of retina showing. Traditional built-in flashes go off near-simultaneously with the shutter – way too fast for your irises to contract. That’s why some newer flashes go off twice: once right before the picture snaps, to make your eyes adjust, and then again to illuminate the scene.

You can also prevent red eye by controlling the angle of the light. Use a separate flash, positioned a few feet away from the camera. And try bouncing the light off a nearby surface instead of pointing it directly at your subject.





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