At some point, you’ve probably heard someone proudly say, “I’ve got 20/20 vision!” But what does that really mean? And what about 20/40 vision? In most states, to get a driver’s license you need to have 20/40 vision or better (with corrective lenses). What is the difference between 20/20 and 20/40? Here we dispel the mystery of the meaning behind the numbers.
20/20 Vision Explained
To start, when we talk about 20/20 vision, we are talking about visual acuity – the clarity or sharpness of your vision. Many people think that having 20/20 vision means “perfect” vision. But really, it’s more accurate to think of it as part of a normal range of vision rather than perfect.
If you have 20/20 vision, it means that if you stand 20 feet away from an eye chart, you can see one of the smaller lines near the bottom, labeled the 20/20 line. When an eye doctor measures your visual acuity, he or she will have you read an eye chart, known as a Snellen chart, from a distance of 20 feet. With 20/20 vision, the top number represents the distance away from the object (eye chart) – 20 feet – and the bottom number represents the distance at which a person with average vision or can read the same line. Put another way, in terms of vision, someone with 20/20 vision is able to perform most everyday activities.
What if your vision is not 20/20? What does that mean? If your vision is 20/30, it means that at 20 feet away from an eye chart, you can read what someone with 20/20 vision can read from 30 feet away. And if you have 20/40 vision, you can read at 20 feet what others with 20/20 vision can read at 40 feet.
The Snellen Eye Chart
The Snellen Eye Chart is the standard chart that most eye care professionals use to test your visual acuity. You probably know it as the chart with the giant E at the top. It shows letters of progressively smaller sizes. The further down you can read the chart, the closer you are to having 20/20 vision. A Snellen chart has 11 lines. If you can read the letters on line 8 of the chart, your visual acuity is 20/20.
How Common is 20/20 Vision?
Only one in three adults have 20/20 vision without any type of correction. Three out of four adults have 20/20 vision with some type of vision correction. One of the goals of wearing glasses, contact lenses, or having corrective eye surgery is to achieve 20/20 vision. Some people may have vision that is better than 20/20, for example 20/15. One is considered legally blind if vision is 20/200 or worse with glasses or contact lenses.
Does Vision Change with Age?
For most people, visual acuity won’t change much with age. Slight changes are possible, but it typically remains fairly consistent throughout life. However, in your 40s and beyond, you may notice a decline in your near vision. As you age, the lens of your eye becomes less flexible and unable to properly bend light. This condition is called presbyopia, and glasses, contact lenses, or other forms of correction will be needed to see clearly across a full range of vision. In your 60s and beyond, cataracts may occur as a natural part of the aging process. In fact, by age 65, most people will have developed changes to the eye’s natural lens, which may eventually lead to the formation of a cataract.
Other Parts of an Eye Exam
Visual acuity is only one thing your eye doctor will measure. Even if you have 20/20 vision, other factors may affect how well you see. For example, the eye chart alone doesn’t measure things like depth perception, peripheral awareness, or contrast sensitivity. Conditions such as cataracts or glaucoma may also affect how well you can see.
Most eye doctors recommend a yearly eye exam. Your doctor will check for signs of certain eye conditions in addition to determining whether or not you need corrective lenses.
References:
https://www.allaboutvision.com/eye-exam/2020-vision.htm
https://www.aao.org/eye-health/tips-prevention/what-does-20-20-vision-mean
