You’ve probably heard people talk about 20/20 vision, but what does it mean? Is it perfect vision? Can you have better than 20/20 vision? If your vision is 20/40 vision, is that better or worse than 20/20? I’d like to take a little bit of time to explain what this weird fraction means.
The first number in the fraction refers to the test distance vision is being tested at. Back in the 1860s, when this method of measuring vision was developed, 20 feet was determined to be the appropriate distance to place a chart to test distance vision. This distance was chosen, because accommodation, or focusing power, is not needed to make objects clear when they are at 20 feet or farther away in the normal eye. The bottom number of the fraction refers to the size of the smallest letter a person is able to see from this distance. For those that like math, the inverse of the fraction correlates to the minutes of arc the detail of the letter subtends.
For everyone else, you can think of the fraction like this. At 20 feet away, the ‘20’ letters can just barely be seen by a normal, healthy eye. As the letters of the chart get incrementally bigger, the numbers get incrementally bigger as well. So, if the smallest letter a person can see is the ‘40’ letter, the smallest letter they can see at 20 feet is the same size as the smallest letter a normal eye can see at 40 feet. This is designated as 20/40 vision. For a person with 20/100 vision, they can just barely see letters at 20 feet that the average eye can just barely see from 100 feet away. When the bottom number is smaller than the top number, it means the person can see better than the average eye. For example, 20/10 vision means a person can see at 20 feet what the average eye would have to be 10 feet from in order to see. This fraction is then referred to as a person’s visual acuity.
Here are some visual acuity milestones measured with this system (Snellen Acuity)
20/8: Theoretically the best vision possible for a human eye to have
20/17: The true average visual acuity
20/20: The acuity determined to be average vision in 1862
20/40: The acuity required for an unrestricted driver’s license
20/70: Lowest amount of visual acuity necessary to obtain a daylight-only driver’s license in Michigan
20/70: Lowest amount of visual acuity necessary to obtain an unrestricted driver’s license in Florida
20/80: Categorized as moderate visual impairment
20/200: Categorized as legally blind
20/400: The ‘Big E’ on most charts
Dr. Andrew Bolles, O.D.