What Causes Red Eye In Photos?

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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Episode Transcript

Speaker 1 (00:00):
I'm Kristin Conger and I'm Caroline Irvan, and we're hosts
of the podcast Stuff Mom never told you that gets
down to the business of being women from every imaginable angle.
That's right. Kristen and I skillfully decode the biology, psychology,
and sociology of ladies and gents from their evolutionary past
a millennial present to better understand all of that stuff.

(00:22):
Mom never told you. No offense moms, Now be sure
to subscribe wherever you get your podcasts. Welcome to brain
Stuff from How Stuff Works. Hi, brain Stuff, I'm Christian Sager,
and I'm here to talk to you about why people's

(00:43):
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. I'm just kidding. It's
it's just simply a reflection. Everything that you can see
is reflecting some amount of light. You can see my
shirt because it's reflecting wavelengths of light and absorbing the

(01:05):
other wavelengths. Black things like my soul or I guess
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 a dark pigment melanin to
promote light absorption. That gives all the photosensitive cells in
the retina the best chance at catching the light coming

(01:27):
at them. The retina contains a lot of those photosensitive cells,
some one hundred and seven 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

(01:50):
illuminate everything within their reach, including the blood vessels in
the retina. A camera with a built in flash will
have that flash pointed direct 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 a minion of zool. Part

(02:11):
of the problem is that you're using a flash you're
in dim light, meaning that your subjects 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 iris is to contract. That's why some newer
flashes go off twice, once right before the picture snaps

(02:33):
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. Check out the brain stuff channel

(02:56):
on YouTube, and for more on this and thousands of
other topics, visit how stuff works dot com.