In American culture, we typically call perfect eyesight “20/20 vision.” We treat it as a bragging right; however, roughly 75 percent of American adults (at least 230 million people) need some kind of vision correction, whether contacts or glasses.
When you head to an optometrist for an annual vision test or to get fitted for corrective lenses, you’ll likely encounter a screen showing letters in descending sizes. Your doctor will ask what you can see with and without assistance. This isn’t just a fun quiz — it’s the Snellen test, also called the 20/20 vision test, and it measures the sharpness of your vision.
Contrary to popular belief, 20/20 vision isn’t actually perfect, and your eye doctor uses many other tests to assess your eyesight.
The vision vernacular
Dutch ophthalmologist Dr. Herman Snellen invented the 20/20 vision test in 1862, to “standardize vision testing,” says Dr. Jennifer Tsai, a New York-based optometrist.
Prior to that, every doctor had their own method of determining how well their patients could see. Many of these were rudimentary at best: One 17-century doctor presented a row of mustard seeds to his patients and measured how far they could step back before they couldn’t count them anymore.
Snellen streamlined these methods with a uniform ratio for measuring eyesight. Instead of mustard seeds, he used standardized characters, called octotypes, and created the distinctive typeface that’s still used today. Snellen’s charts were an instant hit among both doctors and patients. One of his first big orders came from the British army, to accurately test the vision of their recruits.
The 20/20 vision chart measures visual acuity, or the sharpness and clarity of your vision. A patient who can see the Snellen characters clearly from 20 feet away has 20/20 vision. Yet 20/20 vision isn’t perfect; it’s average, normal vision, distinguishing people who don’t need glasses or contacts from the rest.
Here’s how the ratio works: The first number stays constant, since the patient always measures their visual acuity from 20 feet away. The second number compares your vision with this average. For example, if your vision is 20/30, you can read letters at 20 feet that most people can read from 30 feet away. (The 20/20 designation only applies to countries that use the British imperial system of feet. Where the metric system dominates, that standard is 6/6 vision.)
The smaller the second number is, the better your vision. Having 20/8 vision means you could be 20 feet away from the chart and still see it clearly. Someone with 20/20 vision would have to stand eight feet away to see what you’re seeing.
Having 20/15, 20/10 or 20/8 vision is widely sought-after in certain fields. The average Major League Baseball player’s vision is about 20/13.
Still, 20/20 remains the standard because “it’s the average of what people can get to” with glasses or contacts, says Tsai.
More than 20/20
Not every visual acuity test is perfect, or created equal.Some critique the Snellen chart because the spacing between rows and letters varies from chart to chart. Not only that, but since the number of letters on each line differs, visual crowding might interfere with the patient’s results. So why are we using it?
“Even if there are ways that are a little bit better and more accurate, which we use in clinics for research, the 20/20 chart was [always] something that was easy to reproduce and spread,” says Dr. Laura Di Meglio, an ophthalmology instructor at Johns Hopkins University. “People would have to invest a lot to be able to switch over.”
In 1982, the National Eye Institute chart created ETDRS charts. They were originally produced for the Early Treatment of Diabetic Retinopathy Study, a clinical trial that began in 1979. The charts were so successful that the National Eye Institute began requiring them for use in all future clinical trials. These test eyesight at a difference of 13 feet, and present five letters in a single row compared to the Snellen test’s nine.
Perhaps the most well-liked alternative to the Snellen test is the logMAR chart, created at the National Vision Research Institute of Australia in 1976. This chart codifies the distance between the letters themselves and the numbers of letters on each line.Its creators wanted to design a tool “in which the typeface, size progression, size range, number of words per row and spacings … achieve a standardization of the test task.”
That said, your local eye doctor probably will stick to the Snellen test for the foreseeable future. It’s primarily retina specialists and researchers who use the logMAR system. Still, 20/20 vision remains a worthwhile target — whether you were born with it or whether you get there with a little help.