As eye care providers, we often get asked, “What does 20/20 vision actually mean?” You might be surprised to find out the term comes from measuring the sharpness of your vision.
The term “20/20 vision” refers to a person’s visual acuity, or in other words, their ability to read numbers or letters on an eye chart at a standard distance. During a visual acuity test, that standard distance is 20 feet. So if a person has 20/20 vision (or “normal vision”), they can see clearly at 20 feet what should normally be able to be seen at 20 feet. 20/20!
The most common method of measuring a person’s visual acuity is the Snellen chart, which consists of uppercase letters that get increasingly smaller line by line. These are normally used in an “optically folded” exam room, meaning that mirrors are used to help create an optically 20-foot-long room. The rooms are actually much shorter than that. If a patient is unable to read, a system of numbers, symbols, or a “tumbling E” are sometimes used.
The top number is the distance from the chart (in the United States, this is 20ft), and the bottom number is the distance from which someone with “normal” eyesight can read the same line. So, if someone has 20/40 vision, they can see at 20 feet what someone with normal vision can see at 40 feet, and if someone has 20/100 vision, they can see at 20 feet what someone with normal vision can see at 100 feet.
It is, in fact, possible to see better than 20/20. 20/15 indicates that one is able to see at 20 feet what someone with normal vision can see at 15 feet. If a person does not have 20/20 vision, it could be due to a variety of reasons, such as nearsightedness, farsightedness, astigmatism, or an eye disease.
Legal blindness. Patients frequently say that they are, “legally blind without their glasses.” By definition, someone is legally blind if his or her better eye is 20/200 or worse without corrective lenses. One’s peripheral vision is also considered. if someone’s visual field is 20 degrees or less, he or she is considered legally blind.
Presbyopia and 20/20 vision. Another common misunderstanding is when people with 20/20 vision who have previously not needed glasses start developing blurry vision at near around the age of 40. This condition is called presbyopia, and it is a normal part of the aging process. In presbyopia, the lens, which was once soft and flexible, becomes rigid and unable to focus on near objects as well. This phenomenon results in the majority of the population needing glasses for reading at some point, whether they are 20/20 at distance or not.
Perfect vision. 20/20 vision does not always mean “perfect vision.” 20/20 only refers to a person’s visual acuity and what he or she can clearly discern at a distance. It is also measured in a stationary setting and under high contrast conditions. It does not take into account important things like color vision, depth perception, contrast sensitivity, peripheral vision, or how a person’s eyes work together. It also does not automatically indicate good eye health, which is why yearly dilated eye exams are important, even if someone is told they have 20/20 vision.