WEBVTT - How does red eye in photographs work?

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<v Speaker 1>Welcome to Brainstuff from house Stuff Works dot com where

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<v Speaker 1>smart happens. Hi. I'm Marshall Brain with today's question, why

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<v Speaker 1>do people have red eyes? And flash photographs? We've all

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<v Speaker 1>seen red eye where the people in the picture have

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<v Speaker 1>spooky red eyes. These are photos taken at night with

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<v Speaker 1>a flash. Where do the red eyes come from? The

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<v Speaker 1>red color comes from light that reflects off of the

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<v Speaker 1>retina's in our eyes. In many animals, including dogs, cats,

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<v Speaker 1>and deer, the retina has a special reflective layer called

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<v Speaker 1>the tapatum lucid um that acts almost like a mirror

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<v Speaker 1>at the backs of their eyes. The idea is that

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<v Speaker 1>their retina's will reflect light back onto the scene, giving

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<v Speaker 1>these animals better night vision. If you shine a flashlight

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<v Speaker 1>or headlights into their eyes at night, their eyes shine

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<v Speaker 1>back with bright white light. Humans don't have this reflective

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<v Speaker 1>layer in our retinas. If you shine a flashlight in

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<v Speaker 1>a person's eyes at night, you don't see any sort

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<v Speaker 1>of reflection. The flash on a camera is bright enough, however,

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<v Speaker 1>to cause a reflection off the human retina. What you

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<v Speaker 1>see is the red color from the blood vessels nourishing

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<v Speaker 1>the eye. Many cameras have a red eye reduction feature.

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<v Speaker 1>In these cameras, the flash goes off twice, once right

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<v Speaker 1>before the picture is taken, and then again actually to

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<v Speaker 1>take the picture. The first flash causes people's pupils to contract,

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<v Speaker 1>reducing red eye significantly. Another trick is to turn on

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<v Speaker 1>all the lights in the room, which also contracts the pupil.

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<v Speaker 1>Another way to reduce or eliminate red eye in pictures

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<v Speaker 1>is to move the flash away from the lens. On

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<v Speaker 1>most small cameras, the flash is only an inch or

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<v Speaker 1>two away from the lens, so the reflection from the

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<v Speaker 1>flash comes right back into the lens and shows up

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<v Speaker 1>in the photo. If you can detach the flash and

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<v Speaker 1>hold it several feet away from the lens, that helps

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<v Speaker 1>a lot. You can also try bouncing the flash off

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<v Speaker 1>the ceiling if that's an option. Do you have any

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<v Speaker 1>ideas or suggestions for this podcast? If so, please send

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<v Speaker 1>me an email at podcast at how stuff works dot com.

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<v Speaker 1>For more on this and thousands of other topics, go

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<v Speaker 1>to how stuff works dot com.