How does red eye in photographs work?

The flash of a camera is bright enough to cause a reflection off the human retina -- the red we see in these photos comes from the blood vessels in the eye. Learn more about red eyes and cameras in this HowStuffWorks podcast.

Learn more about your ad-choices at https://www.iheartpodcastnetwork.com

See omnystudio.com/listener for privacy information.

Episode Transcript

Speaker 1 (00:00):
Welcome to Brainstuff from house Stuff Works dot com where
smart happens. Hi. I'm Marshall Brain with today's question, why
do people have red eyes? And flash photographs? We've all
seen red eye where the people in the picture have
spooky red eyes. These are photos taken at night with

(00:21):
a flash. Where do the red eyes come from? The
red color comes from light that reflects off of the
retina's in our eyes. In many animals, including dogs, cats,
and deer, the retina has a special reflective layer called
the tapatum lucid um that acts almost like a mirror

(00:41):
at the backs of their eyes. The idea is that
their retina's will reflect light back onto the scene, giving
these animals better night vision. If you shine a flashlight
or headlights into their eyes at night, their eyes shine
back with bright white light. Humans don't have this reflective
layer in our retinas. If you shine a flashlight in

(01:03):
a person's eyes at night, you don't see any sort
of reflection. The flash on a camera is bright enough, however,
to cause a reflection off the human retina. What you
see is the red color from the blood vessels nourishing
the eye. Many cameras have a red eye reduction feature.
In these cameras, the flash goes off twice, once right

(01:26):
before the picture is taken, and then again actually to
take the picture. The first flash causes people's pupils to contract,
reducing red eye significantly. Another trick is to turn on
all the lights in the room, which also contracts the pupil.
Another way to reduce or eliminate red eye in pictures
is to move the flash away from the lens. On

(01:49):
most small cameras, the flash is only an inch or
two away from the lens, so the reflection from the
flash comes right back into the lens and shows up
in the photo. If you can detach the flash and
hold it several feet away from the lens, that helps
a lot. You can also try bouncing the flash off
the ceiling if that's an option. Do you have any

(02:10):
ideas or suggestions for this podcast? If so, please send
me an email at podcast at how stuff works dot com.
For more on this and thousands of other topics, go
to how stuff works dot com.