Eyeglasses or contacts help you have 20/20 vision. But what does that really mean? The phrase “20/20 vision” refers to normal vision, explains J. Kevin McKinney, MD.
“A person with 20/20 vision can see what an average individual can see on an eye chart when they are standing 20 feet away,” says Dr. McKinney, an ophthalmologist and glaucoma specialist at Eye Health Northwest in Oregon City, Ore.
An eye chart measures visual acuity, which is the clarity or sharpness of vision. The top number refers to your distance in feet from the chart. The bottom number indicates the distance at which a person with normal eyesight can read the same line.
For example, if you have 20/30 vision, it means your vision is worse than average. When you are standing 20 feet from the chart, you can read letters that most people see when they are 30 feet away.
A standard Snellen vision testing chart.
However, 20/20 vision is not perfect vision. A person can have 20/15 vision, which is sharper than average. If you have 20/15 vision, you can see a line in the eye chart at 20 feet that the average person can only see when they are 15 feet away. Generally, the goal of correcting vision with glasses or contacts is to bring a person’s vision to 20/20.
Dr. McKinney says that only about 35 percent of all adults have 20/20 vision without glasses, contact lenses or corrective surgery. With corrective measures, approximately 75 percent of adults have 20/20 vision. In almost every state, visual acuity of 20/40 or better is required for an unrestricted driver’s license. People are considered “legally blind” if their corrected vision (with eyeglasses or contacts) is 20/200 or worse.
The American Academy of Ophthalmology recommends that visual acuity in children be tested as soon as a child is old enough to cooperate with an eye exam using an eye chart. Some children lose their 20/20 vision around age 8 or 9, according to Tim Johnson, MD, associate professor of ophthalmology and director of Comprehensive Ophthalmology Services at University of Iowa Hospitals and Clinics. After that age, most people’s visual acuity stays stable. It may decrease slightly as people reach their 60s or 70s.
When you reach middle age, your visual acuity will probably not change, but you may lose your near vision. Your eyes' lenses become less flexible and it becomes harder to change focus from objects that are far away to close objects. This is called presbyopia. You may need reading glasses or bifocals to correct the problem.
Outside the United States, vision is measured in meters instead of feet and the standard is expressed as “6/6.”
Your visual acuity is measured as part of an eye exam. The American Academy of Ophthalmology recommends that you get a baseline eye examination at age 40, the time when early signs of disease or changes in vision may occur.