health history: before eye charts, we used the stars

doctor: “cover your right eye and take a look at the eye chart. can you read the third line for me?”

you: “easy. t, e, m, l, o, p.”

doctor: “great. now, i want you to tell me if a or b is sharper. okay…a or b?”

you: “a!”

doctor: “what about now, a or b?”

you: “…b?”

doctor: “and now, a or b?”

you: “um…” (inner panic) “still b?”

doctor: “a or b?”

you: “a.” “no, b! a?” 

one herman snellen, a dutch ophthalmologist who, in 1862, created the very eye chart that has haunted those of us with poor vision for years. the chart, which would eventually become known as the snellen chart , measures a person’s visual ability from 20 feet away as compared to the average eye.

the eye chart we’re used to seeing. getty

and while you may think that with the now infamous vision test came glasses, it turns out specs existed before 1862. in fact, they were invented in northern italy in the late 1280s, but, without an accurate eye exam, one can only imagine just how useful they were. at the time, eye doctors crafted their own tests, with some featuring letters and words, and others featuring clip art-like images. ancient times were especially creative, with what is now known as the “ arab eye test ,” which involved the “recognition and identification of constellations and celestial bodies of the night sky.” the distance between stars were the basis of the test, showing surprising correlations to the snellen chart, which came about when ophthalmologists learned that vision problems can look different in different people.

this is also, by the way, how the concept of “ 20/20 vision ” came to be. that number is the snellen ratio, which is surmised from how well patients read the chart. so, for example, if you’re able to read the letters eerily accurately, you might have 20/10, which means that you can see at 20 feet what the average person has to be 10 feet away to see. if you can’t read the infamous big e looming at the very top, on the other hand, that means your vision is 20/200 or possibly worse. this means you have to be as close as 20 feet away to see what the average person can see at 200 feet, which could possibly indicate that you are legally blind.

so, how exactly does it work? first, let’s assess the chart. it includes 11 lines of block letters. the first line is the biggest, with just one letter that is typically an e, h or n. the following rows have an increasing number of letters that become smaller in size with each row, so that the final only includes nine. in fact, only nine letters — c, d, e, f, l, o, p, t, z — are used in the entirety of the average snellen chart. the varying shapes of these letters, called optoypes, allow patients to identify verticals, horizontals and diagonals, and are also effective in identifying astigmatism.

the exam itself involves standing or sitting 20 feet away from the chart, and reading each row from top to bottom with one eye covered, and your glasses or contact lenses removed. each eye is tested. the smallest row that you can read with accuracy indicates your visual acuity (a.k.a. sharpness of vision), allowing your doctor to determine if you have problems with seeing at a distance or bringing things that are closer into focus.

while snellen was a visionary, the snellen chart is no cure-all in 2020. over time, optometry has evolved, and now eyes can be tested for everything from how well they perceive contrast, colour and depth, to how healthy they are and whether you’re experiencing glaucoma or retina issues. which means, it is likely that one day snellen’s invention will have become outdated.

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health history: before eye charts, we used the stars

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