Acuity of vision is a term used to describe the clearness or sharpness of your vision when measured at a distance of 20 feet.
Visual acuity is the most common clinical measurement of how your eyes function. It's usually one of the first tests taken during a comprehensive eye exam to determine how sharp your vision is.
If someone has 20/20 vision, it means they can see the same amount of detail from 20 feet away as the average person. If someone has a visual acuity of 20/40, they can see the same amount of detail from 20 feet away as the average person would see from 40 feet away.
Why Is Visual Acuity Important?
Measuring your visual acuity is one of the first procedures you're likely to take during a comprehensive eye exam at an optometrist.
This visual acuity test is a critical part of identifying signs of vision problems. For example, it can detect refractive errors, more commonly known as farsightedness and nearsightedness. Hyperopia(farsightedness) occurs when you have sharp far-range vision while your close-range vision is less clear. Myopia (nearsightedness) is the opposite — your close-range vision may be clear, while your far-range vision is out of focus.
Having 20/20 vision doesn't necessarily mean that your sight is perfect. Visual acuity only measures the sharpness or clarity of your vision at a distance. The strength of your vision also includes eye coordination, depth perception, peripheral awareness, focusing ability, and color vision.
How Is Visual Acuity Tested?
A visual acuity test will examine how well you can describe the details of a letter, picture, or symbol from a certain distance.
Children often take visual acuity tests to keep track of their developing vision. Early detection of these problems can prevent any issues from getting worse.
As an adult, you may need an exam if you're having an issue with your vision or if you feel the clarity of your vision has changed. It can also be part of visual exams you might take to get your driver's license.
There are two types of visual acuity exams.
Snellen
The Snellen test uses a chart that contains letters or symbols. These letters are arranged into rows — each row becoming progressively smaller and smaller down the chart.
To measure visual acuity, you'll be asked to stand approximately 20 feet away from this chart. As you cover one of your eyes, the optometrist or medical professional will ask you to identify the letters in each row and read them out loud. Typically, they will ask you to go down each row and read smaller and smaller letters — until you can no longer identify them. Then, you repeat the process with the other eye.
Random E
The random E test is often used for children or adults who have difficulty identifying letters. Instead of reading out different letters, the random E test only uses a capital letter E.
As you move down the chart, the capital E faces different directions and decreases in size. You'll be asked to identify the direction that the E is facing (up, down, left, or right) until you're unable to see it clearly.
The random E test is calculated in the same way as the Snellen test — the line on which most of the E's can be identified correctly.
Understanding Your Test Results
When you've completed a visual acuity test, you'll get results as a fraction. The top number refers to the distance you stand from the chart. In most cases, this will be 20 feet. The bottom number refers to the distance at which an average person with normal eyesight could read the same line that you read.
Even if you miss a few letters on the smallest line you were able to read, that line will still be factored into your results.
For example, 20/20 is considered normal. 20/40 indicates that the line you read correctly could be read by a person with normal vision from 40 feet away.
Someone with a best corrected visual acuity result of 20/200 is considered legally blind. While it's possible to have vision even more advanced than 20/20 (without a visual aid, like binoculars), the limit is thought to be around 20/10.
How to Read Your Eyeglass Prescription
When you look at your prescription for eyeglasses, you will see numbers listed under the headings of OS and OD. They are Latin abbreviations: OS (oculus sinister) means the left eye and OD (oculus dexter) means the right eye. Occasionally, you will see a notation for OU (oculus uterque), which means something involving both eyes. In general, the further away from zero the number on your prescription, the worse your eyesight and the more vision correction (stronger prescription) you need. A “plus” (+) sign in front of the number means you are farsighted, and a “minus” (-) sign means you are nearsighted. These numbers represent diopters, the unit used to measure the correction, or focusing power, of the lens your eye requires. Diopter is often abbreviated "D."
For example, if your prescription says -1.00, you have one diopter of nearsightedness. This is a fairly mild amount of nearsightedness. If you are -4.25, that means you have 4 and 1/4 diopters of nearsightedness. This is more nearsighted than -1.00 and requires stronger (thicker) lenses. Similarly, +1.00 would be a small amount of farsightedness and +5 would be more.
For people who have astigmatism, there will be three numbers in your prescription. The general form for writing these numbers is S x C x Axis
The S refers to the "spherical" portion of the prescription, which is the degree of nearsightedness or farsightedness discussed above.
The C refers to the "cylinder" or astigmatism, and can be a negative or a positive number. It measures in diopters the degree of astigmatism that you have. The bigger this number, the more astigmatism you have. Astigmatism most often is caused by a cornea that is shaped more like a football than a basketball.
The Axis is a number anywhere between 0 and 180 degrees. It reveals the orientation of the astigmatism. It is not enough to specify how much astigmatism there is; you have to know where the difference in curvature is taking place.
Here are two examples of what prescriptions for eyes with astigmatism could look like:
-2.00 +1.50 x 180
+3.50 +3.00 x 45
The first prescription means that the person has 2 diopters of nearsightedness with 1.5 diopters of astigmatism and an axis of 180 degrees.
The second prescription means that the person has 3.5 diopters of farsightedness, 3 diopters of astigmatism and an axis of 45 degrees.
If You Don't Have 20/20 Vision
It's normal to not have 20/20 vision. Only about 35% of adults have 20/20 vision without using corrective lenses or surgery.
If your visual acuity isn't 20/20, you may need contact lenses, eyeglasses, or surgery. There may also be an underlying eye condition that requires treatment. Your doctor or optometrist can discuss any treatment or correction that might be necessary.
Many people can obtain 20/20 vision with the use of corrective lenses. However, you don't need to have 20/20 vision to live a normal life. In most states, you need at least 20/60 or better in order to get a driver's license.