What is 20/20?
Optometrists love to toss around the term 20/20: “You have 20/20 vison.” or “Gosh, you’re seeing better than 20/20.” Perhaps, you use it at home: “How does this dress look?” “Sorry honey – I don’t see 20/20.” The news program “20/20” was actually named after the visual measurement. Everyone knows 20/20 is ideal, but few actually know what it means.
To know how well someone sees, we must use a standardized system. This affords doctors the ability to know if someone needs more or less correction and to compare the vision between members of a population. For example, it’s means very little to accurately check the vision of an eight month old girl and then record it as “pretty good” or “seems normal.” It’s far superior to be able to record her vision as 20/60 or 20/80. Then visual changes may be tracked with tighter accuracy, her visual acuity may be compared against what’s normal for 8 month olds, and her vision may be corrected or observed against a quantifiable system.
The 20/xx system is also known as the Snellen visual acuity system. The top number refers to the testing distance in feet. Twenty feet is considered to be “optical infinity” due to how our eyes focus and the behavior of light waves at this distance. However, most offices do not have patients sit 20 feet from a chart due to space constraints. Through the use of mirrors and chart calibrations, the shorter distance is compensated for remarkably well.
The bottom number is a bit more complex. It’s the distance at which each element of a letter has an angular height of 1 minute of arc (or 1/60th of a degree). The letter “E” on an eye chart has 5 elements. There is the top bar, a space, the middle bar, another space, and the bottom bar. When one stands 400 feet from the big E at the top of the chart, each of these elements subtends 1 minute of arc. Therefore the size of the letter is called 20/400. For a letter on the 20/20 line, one must stand 20 feet away for each element to subtend 1 minute of arc.
The neurological processing in the human eye and key areas of the brain allows people to easily discriminate between letters with 1 minute of arc (or smaller) features. With a (compensated or actual) testing distance of 20 feet, optometrists therefore embrace the 20/20 line as a perfect endpoint. Please don’t fret if you can’t see 20/20 without glasses or contacts. That will be the subject of a blog post in the near future. Seeing 20/20 with or without visual correction means that at 20 feet away, your eyes see as well as they are expected see. If you can’t read 20/20, please schedule an appointment, and I’d love to help boost your vision.
Spencer Ritenour, O.D.