Speaker 1: Welcome to Brainstuff from house Stuff Works dot com where
Speaker 1: smart happens. Hi. I'm Marshall Brain with today's question, why
Speaker 1: do people have red eyes? And flash photographs? We've all
Speaker 1: seen red eye where the people in the picture have
Speaker 1: spooky red eyes. These are photos taken at night with
Speaker 1: a flash. Where do the red eyes come from? The
Speaker 1: red color comes from light that reflects off of the
Speaker 1: retina's in our eyes. In many animals, including dogs, cats,
Speaker 1: and deer, the retina has a special reflective layer called
Speaker 1: the tapatum lucid um that acts almost like a mirror
Speaker 1: at the backs of their eyes. The idea is that
Speaker 1: their retina's will reflect light back onto the scene, giving
Speaker 1: these animals better night vision. If you shine a flashlight
Speaker 1: or headlights into their eyes at night, their eyes shine
Speaker 1: back with bright white light. Humans don't have this reflective
Speaker 1: layer in our retinas. If you shine a flashlight in
Speaker 1: a person's eyes at night, you don't see any sort
Speaker 1: of reflection. The flash on a camera is bright enough, however,
Speaker 1: to cause a reflection off the human retina. What you
Speaker 1: see is the red color from the blood vessels nourishing
Speaker 1: the eye. Many cameras have a red eye reduction feature.
Speaker 1: In these cameras, the flash goes off twice, once right
Speaker 1: before the picture is taken, and then again actually to
Speaker 1: take the picture. The first flash causes people's pupils to contract,
Speaker 1: reducing red eye significantly. Another trick is to turn on
Speaker 1: all the lights in the room, which also contracts the pupil.
Speaker 1: Another way to reduce or eliminate red eye in pictures
Speaker 1: is to move the flash away from the lens. On
Speaker 1: most small cameras, the flash is only an inch or
Speaker 1: two away from the lens, so the reflection from the
Speaker 1: flash comes right back into the lens and shows up
Speaker 1: in the photo. If you can detach the flash and
Speaker 1: hold it several feet away from the lens, that helps
Speaker 1: a lot. You can also try bouncing the flash off
Speaker 1: the ceiling if that's an option. Do you have any
Speaker 1: ideas or suggestions for this podcast? If so, please send
Speaker 1: me an email at podcast at how stuff works dot com.
Speaker 1: For more on this and thousands of other topics, go
Speaker 1: to how stuff works dot com.