What is visual acuity?
Visual acuity measures how sharp your vision is at a distance. It is usually tested by reading an eye chart.
How is visual acuity tested?
- The Snellen eye chart, with its familiar rows of letters in decreasing sizes, is the most common visual acuity test. While standing or sitting 20 feet from the chart, you are instructed to read each row until you no longer can.
- The Random E test uses a capital letter E that gets smaller in size and also changes direction (up, down, left, right). The test is completed when you can’t tell which direction the E is facing.
- Simplified testing for children. The child is given cards with symbols or letters, which are matched to those on the chart.
You can also test visual acuity at home. But home tests are not meant to replace an exam given by an eye care professional. Your ophthalmologist tests visual acuity as part of a comprehensive eye exam, but others may perform the test, including:
- a pediatrician or other doctor
- a technician or medical assistant
- a nurse
- an optician
Visual acuity can also be tested at schools and most states require testing before issuing a driver’s license.
What do my test results mean?
The results of a visual acuity test are written as a fraction in the U.S. The top number is always the same and signifies the distance between you and the chart, 20 feet. The bottom number is the last line read correctly. For example, a result of “20/20” —normal visual acuity—means you read the line that those with normal vision can read. Visual acuity decreases as the bottom number gets larger. A result of 20/40 means you can see at 20 feet what those with normal vision can see from 40 feet away.
Why would I not have 20/20 vision?
Visual acuity less than 20/20 is common. In most cases, blurry vision from a refractive error (nearsightedness, farsightedness or astigmatism) is to blame. The good news is that glasses, contact lenses or surgery can likely improve your vision to 20/20 or close to it.
Sometimes eye disease, infection or other health problems reduce your visual acuity. In this case, you and your doctor can discuss next steps and what treatment may be needed.
Visual acuity isn’t everything…
We all want uncorrected 20/20 vision, but remember, visual acuity is just one facet of your eye health. With regular eye exams, your ophthalmologist can monitor the entire range of your visual function, including:
- best corrected (with glasses or contact lenses) visual acuity
- peripheral (side) vision
- depth perception (seeing objects in three dimensions)
- eye movement
- binocular (two-eye) function
- the health of the retina, in the back of your eye