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Eye Charts: The Writing on the Wall
Part of your vision test will include a glance at the all-too-familiar eye chart. Read on to learn the philosophy and history behind those shrinking letters.
According to the Seattle Times, the best-selling poster in the United States isn't of Indiana Jones or the cover to Pink Floyd's Generic World Map. It's the eye chart, those iconic rows of decreasingly sized letters that grace optometrists' offices throughout the country. Aesthetics aside, the ubiquitous chart primarily tests visual acuity, which the American Optometric Association summarizes as "the clarity or sharpness of vision." Patients typically stand around 20 feet from the wall, cover one eye, and identify the smallest row of letters they can individually distinguish. Commonly, this boils patients' visual acuities down to a fraction in which the denominator represents how many feet away a person of normal visual acuity could stand while still discerning the letters with the same level of clarity as the patient. In other words, 20/40 vision means the patient needs to stand 20 feet away to make out the same size letters as a person with standard vision can from 40 feet.
These fractions were the brainchild of Herman Snellen, a Dutch ophthalmologist who designed the first popular rendition of an eye chart in the 1860s. The original versions of Snellen's chart included nine letters—C, D, E, F, L, O, P, T, and Z—as optotypes—a term for standardized symbols used to test vision. However, there was room for improvement in Snellen's design; the spacing wasn't quite standardized, and different versions incorporated serif as well as sans serif fonts. Over the years, the Snellen chart has adopted more uniform spacing and cleaner optotypes, and a few alternatives have sprung up for use in other settings. For instance, scientists prefer a chart designed by two Australian optometrists for its logarithmic progression of letter size, and one variation simply orients the single letter 'E' in different directions, making the test easier for patients who are illiterate or unfamiliar with the Roman alphabet.
For all their value, eye charts are still only capable of assessing visual acuity, not vision in general. Full eye exams almost invariably include a staring contest with an eye chart, but optometrists also use different tools to test everything from peripheral awareness and depth perception to focusing ability and color vision.