We’ve all seen photos of people with red demon eyes but do you actually know why this occurs?

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Light enters our eyes through the cornea and pupil to hit our retina which then sends an image to our brains through the optic nerve. At the back of our eyes is a tissue called the choroid in which blood flows to provide oxygen and nutrients to the retina.

Since a camera flash is so quick, often our pupils don’t constrict fast enough to reduce the light the flash produces, which is then reflected back into the camera lens.

The red you see in photographs of someone’s eyes is the reflection of the light bouncing off our blood filled choroids.

Children and drunk people will have this happen more often to them.

Children’s eyes adjust to the dark faster and so will have larger pupils which allow the red reflection to occur.

Drunk people have slower reaction times thanks to the alcohol, and this includes their pupils! Since the pupils are slower to close in response to the flash, you get more blood red eye reflection.

People with blue eyes also tend to have more photographs with red eyes than people with a darker color.

This is not due to the blue color itself but by them normally having less melanin in the layers behind the retina, creating a more reflective background than someone with a darker color. Melanin absorbs light. You can see the difference in photographs of heterochromatic eyes (two different colors), the blue eye will be red while the darker eye is not.

To prevent having red eyes in photographs:

• Avoid using the flash in the first place.
• If you need to use a flash many cameras have an anti-red eye option, which does two flashes – the first to constrict people’s eyes and the second to take the photo afterwards.
• You can also have people not look directly into the camera, as the reflection won’t be as direct.
• The best way to not get red eyes is to have the flash further away from the lens as the light will reflect back towards the flash and not the lens, this is why professional cameras have their flash further from the lens.

Otherwise use software to remove the red eyes from your digital photos.

Bonus Facts:

• If someone continuously shows one red eye and white or yellow in the other in photographs it could be caused by cancer or a disease.

• The creepy glowing eyes of pets or other animals is caused by the reflection of light off their tapetum lucidum, something humans don't have. This special layer of their eyes helps them to see better at night, giving them natural night vision!

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Sources:

• Why Eyes Look Red in Photos -
  and How to Prevent and Fix Red-Eye
• Red-eye effect
• Pictures: 1 2 3 4 5

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