Now See This: Understanding Your Visual Acuity

When your optometrist performs a routine eye exam, one of the things that he or she is evaluating is your visual acuity. Find out what visual acuity is, how it is tested, how it is measured, and how your assessment is used to correct any visual deficits that you may have.

Can You Read the Chart?

Visual acuity describes how clearly you can see things. Visual acuity is dependent on how well your eyes process light to enable the retina of each eye to focus on the images that you are looking at, such as lines of text on the Snellen chart. This chart is one of the tools used by optometrists to evaluate visual acuity. The Snellen chart is standardized, and it exhibits rows of letters. The top row displays one large letter, and each row underneath displays more and more letters in progressively smaller text. To perform this test, your optometrist will ask you to cover one of your eyes and read the row that displays the smallest letters that you can see clearly. You will then cover your opposite eye and repeat the reading. The row with the smallest text that you are able to see accurately will determine the measurement of your visual acuity. One eye may see more clearly than your other eye, which is why each eye receives its own score.

What Does Your Visual Acuity Score Mean?

If your optometrist announces that you have a visual acuity score of 20/20 in both eyes, you can sigh with relief knowing that you have normal vision, but what do those numbers actually mean? The first digit of 20 represents the distance between you and the Snellen chart, which is posted 20 feet away from where you are seated to take this test. If your optometrist practices in a smaller office setting, the chart is scaled down appropriately so that the text appears the same to your eyes as it would if you were reading the standard sized chart at 20 feet away. The second digit refers to the row with the smallest letters that you were able to read clearly. Each row of text on the Snellen chart has a corresponding number, and the row with the smallest text has the lowest number, which is 20. If you have 20/20 vision, you were able to clearly read the smallest row of text at a distance of 20 feet. The second digit also represents the distance in feet at which someone with normal vision can clearly see the same row of text. If you have 20/20 vision, then you can see a row of text at a distance of 20 feet away from the chart with the same clarity at which a person with normal vision can see that same row at 20 feet away.

In order to obtain a pilot's license, pilots must possess a visual acuity of 20/20. In most of the 50 states, 20/40 vision is the minimum visual acuity requirement for attaining an unrestricted driver's license. If you have 20/40 vision, you will not be pursuing a flying career or hobby, but you will be able to drive your car. With 20/40 vision, you can see an object clearly at 20 feet away, but you cannot see it clearly at 40 feet away, whereas someone with normal vision can. A person with a visual acuity of 20/200, meaning that he or she cannot clearly see objects beyond 20 feet away that someone with normal vision can see clearly at 200 feet away, is considered to be legally blind

Will Further Testing Be Needed?

The Snellen chart is just the beginning of a series of tests that your optometrist will perform to assess other aspects of your visual acuity. Achieving a visual acuity score of 20/20 on the Snellen chart test only tells your optometrist how clearly your eyes can see letters, shapes or objects from a distance. To get a complete picture of your visual abilities, peripheral vision and depth perception need to be evaluated as well. Your optometrist will position a phoropter in front of your face. This device resembles a robotic mask from a science-fiction film. It has spaces for your eyes to look though, and a series of dials. These dials are refracting lenses, and as you look through the phoropter, the optometrist flips the dials to provide several different presentations for viewing objects. Each time the lenses change, you will be asked to perform certain tasks based on what you see. Some examples of such tasks include the following:

  • Stating which of your eyes sees a letter more clearly
  • Comparing how clearly or blurry images appear as the lenses are changed
  • Stating when you see two moving objects line up perfectly

This test is called the refraction assessment, and it tells the optometrist a lot about how efficiently the parts of your eye are working to enable you to see and how well your eyes are seeing. Once you have read the Snellen chart and sat through the refraction assessment test, your optometrist will determine if you have any visual deficits that need to be corrected.

Do You Need Visual Correction?

If your optometrist notes any visual deficits, he or she will discuss a plan to correct your vision. Some commonly diagnosed visual deficits include the following:

  • Hyperopia, or farsightedness, which means that you can see objects clearly at a distance, but they appear blurry when you view them in close proximity
  • Myopia, or nearsightedness, which means that you see objects clearly when they are close, but they appear blurry when you view them at a distance  
  • Astigmatism, which means that objects appear blurry to you whether you view them close up or at a distance 
  • Presbyopia, which means that as you are getting older, you are experiencing an age-related deterioration of your visual acuity

Your optometrist will recommend corrective lenses, which include eyeglasses and contact lenses, and he or she may also discuss the option of corrective laser surgery to provide you with 20/20 visual acuity. Laser in-situ keratomileusis, or LASIK, surgery has benefitted patients diagnosed with myopia, hyperopia, and astigmatism.  

Early detection of a decrease in your visual acuity will enable you and your optometrist to take the necessary steps to improve your vision, which will relieve eye strain and preserve your ability to see and carry out your daily tasks for a brighter and clearer quality of life.