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Y’know that thing where your eyes glow bright red in photos?
It’s called the red-eye effect.
But it doesn’t mean you can secretly fire laser beams from your eyes or anything.
Instead, you can blame the bright flash of a camera, and how your eye adapts to changes
in light.
Your eyes are basically fluid-filled orbs that can detect light and send messages to
your brain, so you can see images.
Light passes through a thin membrane called the cornea, then a hole called the pupil,
and then a lens – so it can be focused, and absorbed by photoreceptor cells in the
back of your eye.
Your pupil is surrounded by this muscle-filled structure called the iris, which is the colored
part of your eye.
You don’t consciously move these muscles, but they control how big your pupil is, and
how much light you let in.
When it’s dark, your iris makes the pupil wider to let more light into your eye.
But when there’s bright light, your iris makes the pupil smaller.
So, the red-eye effect usually happens in a dark environment, when your pupils are really
wide.
Normally, all the light that enters your eye is absorbed by photoreceptors, or by a pigment
called melanin in a tissue layer in the back of your eye – the same pigment that influences
the color of your iris, hair, and skin.
And since all the light’s absorbed, your pupils look black.
But if there’s suddenly a bright camera flash, all that light floods into your eyes
before your iris muscles have time to contract.
Some of it might reflect off the blood vessels in the back of your eye, and shows up as a
glowing red light to ruin that group photo you were trying to take.
To fix this, some cameras make a couple quick flashes of light before the actual flash when
the photo’s taken, so that your iris muscles start contracting, and let in less light.
You could also try brightening the room, so your pupils aren’t as wide in the first
place, or avoid looking directly at the camera lens.
The red-eye effect is pesky, but it isn’t always bad.
It can actually be a handy tool for diagnosing eye problems.
If someone was looking directly at the camera lens in dim lighting, and they have a glowing
white or yellowish eye, there might be an infection, some cancerous cells, or those
blood vessels might be twisted or leaking.
Sometimes photos will only have one glowing red eye, which might mean someone’s eyes
aren’t looking in the exact same direction.
Or, it could mean there’s different amounts of melanin in the back of each eye, so they’re
different colors.
So mostly the red-eye effect is just annoying, but you might want to keep an eye on it.
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