What is 20/20?

Written by David Evans, PhD, MBA   Last modified on August 6, 2018

“20/20” lingo is practically part of our culture these days, but how many people actually know what it means? A myriad of television shows and companies — with nothing to do with eye care — call themselves “20/20 this” and “20/20 that.”(To get my drain fixed last week, I called 20/20 Plumbing.) The expression is clearly seen as a superlative, like “hindsight is always 20/20.” So it might surprise you learn that it’s really not a superlative, it’s the average.

The first standardized eye chart was developed in 1862 by Dr. Herman Snellen, a Dutch ophthalmologist from Utrech, Holland. Dr Snellen wanted to standardize a test for both near and distance vision; so he developed specific letter sizes in block formats, such that the letters had specific angular widths whether viewed near or at a distance. As his test quickly became the worldwide standard, studies began to determine what size letters could be read by the average person. Some speculate that during vision screenings at the World’s Fair held in New York in 1924, enough people were tested with Dr. Snellen’s chart that the true average could be established.

Data shows that the average person can read a block letter that is the size of 1/12th of a degree. Since most eye tests were given at a distance of 20 feet, the nomenclature of 20/20 was born to describe the vision of a person who could read the average size letters from a distance of 20 feet. In Europe, where they use the metric system, it is called 6/6, because the eye tests are given at a distance of 6 meters.

If a person has to be closer to the average size letter to read it, then the bottom number changes in inverse proportion to the distance. For example, if a person has to be 20 feet away to read what the average person can read at 40 feet, he or she has 20/40 vision.

Interestingly, the goal of many of the original laser eye surgery technologies was that patients would be able to see 20/20 without glasses or contacts after the procedure. Now, with wavefront laser technologies and highly advanced pre-screening tests, it is not unusual for patients to see better than the average 20/20. Many achieve 20/16 vision, and in some cases 20/10.