What Does 20/20 Vision Actually Mean?
Not sure what 20/20 vision means? You're not alone. We'll explain what those numbers mean and how your optometrist can help you if you don't have 20/20 vision.
What Is 20/20 Vision?
Optometrists use the term "20/20" vision to describe visual acuity, or your ability to see clearly without contact lenses or eyeglasses. If you're like the the average person, you can see an object clearly at 20 feet away without contact lenses or eyeglasses.
The greater the second number, the blurrier your vision. For example, if you have 20/200 vision, others can see a tree clearly at 200 feet, but the leaves and trunk only come in to focus when you're 20 feet away. If the second number is lower than 20, your vision is better than average. Let's say your vision is 20/15. In that case, you see an object clearly at 20 feet while an average person must stand 15 feet away to see the same object clearly.
How Is 20/20 Vision Measured?
It wouldn't be practical to measure your eyesight by asking you to stand outside and move back and forth in front of a tree. That's why eye doctors developed eye charts like the Snellen chart used today. Originally, patients stood 20 feet away from the chart while reading letters that got progressively smaller on each line. Today, mirrors make it possible to measure your vision in a smaller space.
Each line on the chart corresponds to a specific acuity measurement, like 20/20 or 20/40. As you read each line, your optometrist notes if you struggle to see any of the lines or letters.
Your eye doctor may also also ask you to look into a auto refractor when determining your visual acuity. During this test, a computer estimates your acuity by measuring how light reflects off your retina.
Can You Still Have Good Vision If You Don't Have 20/20 Vision?
Many people with in the U.S have refractive errors that affect their vision. Refractive errors affect the way light bends and focuses on light-sensing retina at the back of the eye. These four refractive errors can affect visual acuity:
- Myopia. Myopia, the most common type of refractive error, affects 42% of Americans, according to the International Myopia Institute. Myopia, or nearsightedness, occurs when light entering the eye focuses in front of the retina instead of directly on it. This can happen if your eyeball is too long or the cornea is too rounded. Nearsightedness can also be caused by an issue with the lens inside your eye. If you're nearsighted, you can see near objects clearly, but objects in the distance are blurry.
- Hyperopia. Hyperopia, or farsightedness, happens when light is focused behind the retina. Farsightedness can be caused by a too-short eyeball, a cornea that isn't curved enough, or an imperfection in the lens of the eye. If you have hyperopia, you can see things in the distance ,clearly but struggle to read or look at close-up objects.
- Astigmatism. Astigmatism occurs due to an irregularly curved cornea or a lens imperfection. These irregularities affect how light enters the eye, causing blurry vision at near and far distances.
- Presbyopia. Presbyopia, an age-related refractive error, happens when the lens becomes less flexible. The lens changes shape when you change your focus from near to far. People who have presbyopia struggle to see near objects without reading glasses.
Eyeglasses and contact lenses help people see well even if they don't have perfect vision. These devices correct your vision to 20/20 vision or as close to this number as possible while you're wearing them. Both devices change the ways light is refracted as it enters your eyes and keeps it focused precisely on your retinas.
Although your eye doctor will have rough idea of your prescription from your Snellen and autorefractor results, he or she must fine-tune your prescription to ensure that your vision is as crisp and clear as possible. During this part of the visit, you look into a phoropter, an instrument that contains lenses of many different strengths. As you look into the instrument, your eye doctor flips different lenses in front of your eyes and ask you to pick the clearer of two options. The test helps determine the perfect prescription for you.
Have things been looking a little blurry lately? We can help you improve your vision. Contact our office to schedule an appointment.
Sources:
International Myopia Institute: 2023- 4 IMI Myopia Attitudes and Practices Survey
https://myopiainstitute.org/myopia/
All About Vision: What Tests Are Part of an Eye Exam, 1/10/2020
https://www.allaboutvision.com/eye-exam/parts-of-eye-exam/
American Optometric Association: Visual Acuity
https://www.aoa.org/healthy-eyes/vision-and-vision-correction/visual-acuity?sso=y
American Academy of Ophthalmology: What Does 20/20 Vision Mean?, 1/28/2022
https://www.aao.org/eye-health/tips-prevention/what-does-20-20-vision-mean