It's common to hear “20/20 vision” when describing good eyesight, but you may not know what the term means.
20/20 vision is a measure of one's visual acuity and indicates how clearly we see objects on an eye chart at 20 feet. The eye chart contains images, usually letters or pictures, of a very specific size gradually getting smaller near the bottom of the page.
How a Healthy Eye Works
To perform the test, you'll cover each eye and identify letters on an eye chart positioned 20 feet away (or simulated with mirrors to look 20 feet away). Your eye doctor will ask you to identify the smallest letters you can read with each eye separately and then with both eyes together.
The smallest letters you can read determine your distance vision, or visual acuity.
Normal Vision at 20 Feet
If your visual acuity test determines you have 20/20 vision, it means you can clearly see what a person with normal vision should be able to see from 20 feet away. If you have 20/40 vision, you can see at 20 feet what a person with normal vision can see at 40 feet.
If you're one of the few people with 20/15 vision, you can see at 20 feet what a person with normal vision sees at 15 feet, and so on.
In the U.S., you may be considered “legally blind” if your best-corrected vision (distance vision while wearing glasses or contacts) is 20/200 or worse. To get a driver's license in the U.S., your best-corrected vision usually must be at 20/40 or better. If that requires glasses or contacts, it may say so on your driver's license, and legally you may be required to wear them at all times when driving.
Just One Part of Your Vision
While your visual acuity at 20 feet helps determine whether you need glasses or contacts for distance vision, it's only one part of your vision. Other parts include:
- Close-up vision or “near vision”
- Depth perception
- Peripheral vision (seeing to the sides)
- The ability to see colors
In addition, some eye conditions may develop very gradually and not have noticeable symptoms, so it's important to have regular eye exams even if you think you have 20/20 vision and that your eyes are healthy.