Astigmatism is a condition that affects the shape of a person's cornea (the outermost clear layer of the eye). Rather than being a uniform round shape, the cornea is more of an oval in individuals with astigmatism.
This odd shape causes light to refract strangely when it enters the eye, and can lead to some pretty weird optical effects.
Twitter user @UnusualFacts6 shared a side-by-side comparison of the way people with astigmatism see light and the way those with a typically shaped cornea see it.
There's a pretty marked difference.
The photos caused quite a few people to question if they might have an astigmatism. This led to a few existential crises.
Refractive errors, which include myopia and hyperopia (nearsightedness and farsightedness, respectively) in addition to astigmatism and another condition called presbyopia, are estimated to affect around half of all people in the US who are 20 years-of-age or older.
There were several people who thought the first photo was the normal one.
Apparently astigmatism is the cause of some arguments, too.
Most refractive errors can be corrected with prescription lenses (either glasses or contacts), but they don't always fix everything.
Those with astigmatism have to order special contact lenses if they want to go that route, often referred to as "toric lenses", that are designed to fit the unique shape of their eyes. These are often more expensive than typical contacts, as they are harder to mass-produce.
Astigmatism is, thanks to modern technology, something that usually isn't difficult to live with. If @UnusualFacts6's post has you thinking you might have the condition, it is worth talking to your doctor about it; a good pair of glasses can make a world of difference.