Red eyes might ruin your photos, but at least they let you know everything's in full working order.
People were pretty creeped out when they discovered the reason why their pupils change colour when photographed with the flash on, however, it's a lot better than the alternative.
If you weren't aware, cameras basically get a really good look into your soul - and what's going on behind the eyes - when your taking well-lit photos with your friends.
The flash effectively lights up the inside of our eyes and gives us a glimpse of the juicy red blood vessels that sits behind them, causing the red effect.
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According to the American Academy of Ophthalmology: "A red reflex happens when the flash of a camera lights up the blood-rich retina.
"If the eyes are looking directly at the camera lens and the colour of the reflex in both eyes is red, that's usually a good sign that the retinas of both eyes are unobstructed and healthy."
But some people have scrolled through their amateur photoshoots and realised that their pupils aren't affected by the photography phenomenon, and sometimes it ends up saving their lives.
Don't get me wrong, there are sometimes simple explanations for why your eye might not be red in one photo out of 100 - maybe you weren't looking directly in the camera lens, or you blinked just as the image was taken.
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However, the lack of red eyes could also be a warning sign that something isn't quite right with your peepers.
There are a few common conditions that can be picked up on in this way, such as strabismus, which means a person's eyes are pointing in different directions.
Also known as a squint, this is usually a minor issue regularly seen in young children, which can be corrected and could explain why only one eye is red - as the other pupil was not looking directly at the camera.
But this could also be a symptom of a rare type of childhood eye cancer called retinoblastoma, which has famously been detected by a host of parents who picked up on the fact their kid's eyes looked strange in flash photographs.
It affects the back of the eye, the retina, which sends signals to the brain to help you see and is life threatening.
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However, most cases are usually successfully treated when it is found early on, so make sure you keep snapping as many pictures as you can to keep an eye on the youngsters in your life.
It may look like a white, yellow or black reflection on the pupils and may occur in both or one eye - so consult an optician or your doctor if you have any concerns.
Those who spot whiteness should also be wary of cataracts or retinal detachment, as this could be a sign of it.
If you notice a white or yellowish glow, known as leukocoria, this could indicate that someone is suffering from the painless and slowly progressive eye condition, Coats' Disease.
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According to the NHS, this is an 'abnormality of the blood vessels in the back of the eye', also known as the retina.
It typically only affects one eye, but can result in the permanent loss of vision if left undiagnosed.
A strange cast over your eye in a snap could be something and nothing, but it's worth getting checked out to be on the safe side.
Olivia Burke