You already know that thing where your eyes glow red in photos. Yeah, the red-eye effect, which happens usually when camera uses flash light in the dark environment. But what causes your eyes to appear red in photos sometimes?

In this episode of SciShow’s “Why Does My Eyes Glow Red In Photos?” Michael Aranda explains that this ‘pesky’ phenomenon called the red-eye effect is actually a result of your eyes effectively adapting to changes in light. Your eyes are basically fluid-filled balls of sphere that can detect light and send messages to your brain via the optic nerve, so you can see images.

Light rays travel through a thin membrane called the cornea, and then pupil of the eye, so that it can be focused, and absorbed by photoreceptor cells in the back of your eye. The iris, which forms the colored portion of the eye, controls the size of your pupil which in turn controls how much light you let in. When it is dark, your iris makes the pupil dilate and let more light to enter into your eye. But when there is bright light, your iris makes the pupil constricts and allows only a small amount light to enter. So, the red-eye effect usually happens at night and in dim lighting condition, 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,” explains Micheal. “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.”

In dark environments, when a camera flash goes off, your iris muscles, which control your pupil, don’t have time to contract fast enough to reduce the amount of light entering the eye. As a result, the light reflects off the blood of the choroid – the vascular layer of the eye, containing connective tissue that nourishes the retina. Moreover, due to ample supply of blood in the Choroid, the camera picks up the red reflection, giving you the red-eye effect. The darker the environment you are in, the wider your pupils get, and thus the greater the chance of having red eyes in photographs.

The quantity of melanin pigment in the iris is also responsible for causing the red-eye effect. People, especially light-skinned with blue eyes, tent to have lower levels of melanin. Their eyes absorb less and reflect more light and therefore more likely to get red eye in photo compared to dark-skinned people with brown eyes. Children also tend to have red eyes more commonly in photographs compared to adults. This is because their pupils dilate faster, and hence increased dark adaptation.

Red-eye effect can be fixed. Some cameras even make few preliminary flashes before the actual flash when the photo’s taken to give the iris muscles enough time to contract and let in less light. Also brightening the room or avoiding direct contact with the lens can also help.

“The red-eye effect is pesky, but it isn’t always bad. It can actually be a handy tool for diagnosing eye problems,” says Michael. “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.”

Some people sometimes will only have one glowing red eye. This could mean that they aren’t looking in the exact same direction or they have strabismus, abnormal alignment of one or both eyes. This could also be because there is different concentrations of melanin in the back of each eye.

Interestingly, dogs, cats and many other animals may not have red eyes in photographs but other colors. For example, the eyes of dogs and cats glow either green or blue in photo. This is because they have a tapetum lucidum – a reflective layer of the choroid, which reflects light back onto the retina to help them with night vision. Since tapetum lucidum is situated in front of the choroid, it reflects green (or blue) light that would have normally been absorbed.