What Is a Visual Acuity Test And What Does 20/20 Mean?

How can you tell if you have good vision? This question is more complex than it appears. Ophthalmologists have a number of ways to test eye function, including depth perception, color vision, and peripheral vision.

Despite the range of options, visual acuity — the sharpness or clearness of one’s eyesight — is considered the most common clinical way to measure a patient’s vision.

A visual acuity test is a way to measure the sharpness of someone's vision from certain distances.

If you’ve ever visited an eye doctor, you’ve likely taken a visual acuity test. The procedure is performed as a part of standard eye exams, checkups, and contact lens appointments.

During a visual acuity test, patients typically read a chart of letters or symbols to measure how clearly they can see objects and symbols from specific distances.

The two most common visual acuity tests are The Snellen Test and the Random E Test.

The Snellen Test is the most common way to test visual acuity. During the test, an ophthalmologist shows the patient a chart with rows of capital letters of differing sizes. The largest letters make up the top row, with the letters on each row getting progressively smaller.

As the exam begins, patients cover one eye and attempt to read the letters on each row — usually from 20 feet away from the chart.

Patients will start with the top row of largest letters and move down to the smaller letter rows until they can no longer accurately read the chart. The eye doctor uses this information to score the patient’s visual acuity.

The Random E Test is less common than the Snellen Test, and it’s normally used for young children and adults who have trouble seeing letters.

During the test, the eye doctor projects images of a capital E facing different directions. Patients stand 20 feet from the image and must identify which direction the “E” faces.

As the test progresses, the E becomes smaller and more difficult to discern. The ophthalmologist will measure visual acuity based on the last “E” the patient is able to identify correctly.

When you’re finished with a visual acuity test — either Snellen or Random E — your eye doctor will give you the results in the form of a fraction, such as 20/15, 20/20, or 20/30.

The top number — numerator — refers to the distance the patient stands from the chart. The standard for eye exams in the US is 20 feet.

The bottom number — denominator — refers to the distance at which someone with normal vision can read the line of letters or symbols. For instance, if you have 20/40 vision, the letter size you would read at a distance of 20 feet, a person with ‘normal’ vision could read from 40 feet away.

A score of 20/20 on your visual acuity test means you have normal vision. More specifically, it means you have normal visual acuity standing 20 feet away from an object.

If the denominator on your visual acuity test is above 20 — i.e.30 or 40 — it means you have a below-average vision. In most states, you need above 20/40 vision to receive a driver's license, and those with 20/200 vision are considered legally blind.

Around 35% of adults have 20/20 vision without the aid of glasses or contacts. The number goes up to 75% when you take into account people who’ve had some form of vision correction.

For instance, if someone has 20/15 vision at 20 feet, they can read a line of letters an average person can see from 15 feet. Thus, the person with ‘average’ or ‘normal’ vision would have to get closer to the chart in order to see the same sized letters.

While people can have above 20/20 vision, there appears to be a limit to visual acuity. Most experts believe the natural limit to visual acuity is around 20/10.

If you don’t score 20/20 on your visual acuity test, you’re not alone. Most adults don’t have 20/20 vision without some form of vision correction. For those with worse than 20/20 vision, there are several ways to get your visual acuity near normal.

Purchasing corrective lenses such as glasses or contacts is the most common way to correct refractive errors and enjoy normal vision. To receive a glasses or contact prescription, schedule an appointment with an eye doctor.

With a glasses prescription, patients can choose from different styles and lenses suited for specific activities like reading and driving. For contacts, there are a variety of lens types to choose from depending on patient preferences.

Patients who wish to enjoy clear vision without the aid of glasses or contacts may also consider a vision-correcting eye surgery. The two most common are LASIK — best for patients 21 - 40 years old — and Refractive Lens Exchange — ideal for patients over 40.

Many patients who undergo laser refractive surgery such as LASIK will be able to see better after surgery than they could beforehand with glasses or contacts. While they may have 20/20 vision with glasses or contact lenses, many patients will achieve a visual acuity of 20/15 or 20/10 after surgery.