Measuring Vision Accuracy
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
For people with 20/20 vision, light focuses perfectly on the retina to provide clear vision. Light must first pass through the cornea and pupil and then be perfectly focused by the lens onto the retina.
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 indicate your distance vision, or visual acuity.
Normal Vision at 20 Feet
An eye chart measures the sharpness of your vision. The results are given with two numbers. Average vision is shown as 20/20. The first number refers to your distance, in feet, from the chart. The second is the distance at which a person with normal eyesight can read the same line.
So, if you have 20/30 vision, it means your vision is worse than average. When you're standing 20 feet from the chart, you can read letters that most people see when they're 30 feet away. If you have 20/15 vision, this means your vision is better than average because you're able to see a line on the chart at 20 feet that an average person would only see at 15 feet away.
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, keep in mind that some vision problems have no noticeable signs or symptoms. So it's important to have regular eye exams. Early detection and treatment of eye conditions can help prevent vision loss.