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People associate the numbers “20/20” with normal vision, but what does it really mean to have 20/20 vision?
20/20 is a term to describe normal visual acuity. Visual acuity is how clearly you see at a specific distance. For example, when your eye doctor asks you to read letters off a chart during an eye exam, they are measuring your visual acuity.
With a measurement of 20/20, the first number indicates the test distance. In other words, how far you are standing from the eye chart. In this case, the first 20 refers to a test distance of 20 feet. This number usually does not change, unless you are using a different eye chart and test distance.
In other countries, many optometrists use a test distance of 6 meters. The equivalent to 20/20 is 6/6.
The second number varies depending on how small of a letter you can see. For example, maybe the smallest letter you can read on the chart corresponds to 20/40. This means someone with normal vision can see the letters at 40 feet, but you can only see those same letters at 20 feet.
The larger the second number, the poorer the visual acuity. If you are diagnosed with a refractive error, such as astigmatism, myopia (nearsightedness), or hyperopia (farsightedness), you will need corrective lenses to fix the vision problem.
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Most ophthalmologists use the Snellen eye chart to measure visual acuity. When you view the eye chart, you may notice that different letter sizes correspond to varying levels of visual acuity.
A Snellen vision test starts with the biggest letter at the top, descending to the smallest letters at the bottom. Typically, the largest letter corresponds to a 20/200 letter size, while the smallest letters correspond to 20/10 letters.
Since a standard Snellen visual acuity chart is viewed at 20 feet, this type of chart is difficult to fit into a typical exam room. Many eye doctors use mirrors to simulate a 20-foot distance by reflecting the eye chart in the mirror. This method is more practical than constructing a room 20 feet in length. Digital eye charts are a more modern solution to this problem.
When you have an eye examination, the eye doctor measures your visual acuity without glasses or contact lenses, called uncorrected visual acuity. Then, they perform a refraction to see what your visual acuity is with your eyeglass prescription, which is your corrected visual acuity. This is the test most people are familiar with, when your doctor asks, “Which one is better, one or two?”
Estimates show at least 93 percent of people over age twelve in the United States have 20/40 vision or better (uncorrected).
If a patient cannot read the largest Snellen letters on the chart, the optometrist will perform additional visual acuity tests, such as:
20/20 means you have good visual acuity, but other factors contribute to the overall quality of vision:
All of these visual components are necessary to have normal, functional vision.
Some people can see better than 20/20. If your vision is 20/15, this means you can see one line of letters smaller than 20/20 on the eye chart. 20/10 vision is one line lower than 20/15, which is the smallest letter size most eye charts have.
Younger patients are more likely to see better than 20/20 versus older patients. Our visual acuity tends to decline with normal age-related changes in our eyes.
Your eye doctor can perform a refraction to determine if eyeglasses or contact lenses can correct your vision to at least 20/20. If you cannot be corrected to 20/20 or better, or if you feel your quality of vision in glasses or contacts is not ideal, LASIK eye surgery may be an option.
Some people have minor irregularities in their vision called higher-order aberrations. Most of these aberrations occur in your cornea, the clear tissue over the front of your eye. These irregularities cause light-related distortions such as:
While regular glasses and soft contact lenses improve your visual acuity, they cannot correct higher-order aberrations. Rigid gas permeable contacts or digital high-definition glasses may help minimize higher-order aberrations.
However, a wavefront-guided LASIK procedure can reshape your cornea to correct higher-order aberrations. This customized LASIK procedure can deliver higher quality vision to patients who are dissatisfied with glasses and contacts. Many patients can see better than 20/20 after their LASIK surgery.
Consult an eye surgeon to find out if you have higher-order aberrations and see if wavefront-guided LASIK is right for you.
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“Visual Acuity: What Is 20/20 Vision?” American Optometric Association, www.aoa.org/patients-and-public/eye-and-vision-problems/glossary-of-eye-and-vision-conditions/visual-acuity. Accessed 6 Mar. 2020.
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