What Is Visual Acuity?

Reviewed by Brian Boxer Wachler, MD on November 7, 2023

What Does Visual Acuity Mean?

You may have heard the term "visual acuity" when getting your eyes checked. But what does it mean?

Visual acuity refers to the clarity or sharpness of your vision. It measures how well you can see details, such as letters or objects, at a certain distance.

Expressed as a numeric value, your visual acuity helps eye care professionals determine if you need vision correction or if your current prescription may have changed. It can also indicate visual impairments or common eye conditions such as nearsightedness (myopia) and far-sightedness (hyperopia).

Certain tests may be performed to evaluate the sharpness and clarity of your vision. Read on to learn more about visual acuity and how it’ is measured.

When is a Visual Acuity Test Performed?

A visual acuity test is often performed during a comprehensive eye exam with an optometrist or ophthalmologist. These tests are provided for adults and children and may be recommended more frequently if you have risk factors such as a family history of eye diseases.

The American Optometric Association (AOA) recommends that adults have eye examinations on an annual basis. Children should receive an eye exam between the ages of 6-12 months and again between 3-5 years old. They should then undergo an exam before starting the first grade and once per year following that.

Types of Visual Acuity Charts

An eye chart is a tool used to assess visual acuity. These charts help your eye doctor evaluate your vision based on how well you can identify specific objects, patterns, or letters at a given distance. The most common eye charts include:

Snellen Chart

The Snellen eye chart is the most widely used chart to test visual acuity. It consists of several rows of uppercase letters that decrease in size as they go down the chart. The top row usually contains a single large letter "E.” The participant reads the letters out loud from the smallest row they can see clearly.

Random E Chart

The Random E chart is similar to the Snellen chart but consists of a series of capital "Es" arranged in different orientations. This chart allows an eye doctor to test visual acuity in people who can’t read letters, such as young children, or those unable to communicate verbally.

Tests for Children

Some charts are designed for children or toddlers who don’t know their letters yet. These charts feature pictures of familiar objects instead, making it easier for children to understand and participate in the exam.

One example is a Cardiff visual acuity test, which involves a set of cards with easily recognizable shapes (such as a fish, dog, or train) positioned at the top or bottom. During the test, the eye doctor watches the child’s eye movements to determine which part of the card they look toward. This can help the eye care professional assess the child’s visual acuity.

What Happens During a Visual Acuity Test?

During a visual acuity test, you’ll sit or stand a specific distance away from the eye chart. The standard for adults is 20 feet, while children may be tested at 10 feet. Some devices use mirrors so you can be tested by looking into the machine.

You’ll then cover one eye at a time and read the letters out loud from each row. You’ll be asked to start at the top and work your way down until the letters become too difficult to read. The smallest row of letters you can read is used to help determine your visual acuity.

An eye care professional may perform this test on a patient with and without glasses. Testing for "corrected" and "uncorrected" vision helps determine if your current prescription is still effective or if you might need a new one.

What Do Visual Acuity Numbers Mean?

Visual acuity results are expressed as a fraction, such as 20/20. The first number represents your distance from the eye chart in feet. The second number represents the distance (in feet) at which a person with normal vision can clearly see the same line of letters. Here are some examples of these measurements and what they mean.

20/20 Vision

20/20 is the standard for normal vision. If you have 20/20 vision, you can see clearly at 20 feet what a person is expected to see at that distance. Contrary to popular belief, 20/20 does not mean you have perfect vision. This is because visual acuity doesn’t include peripheral vision, depth perception, or color vision, among other aspects of vision.

20/40 Vision

If you have 20/40 vision, you can see at 20 feet what a person with normal vision can see at 40 feet. In other words, your vision is only slightly less than what is considered standard. In most states in the U.S., the minimum visual acuity score required for an unrestricted driver's license is 20/40.

20/60 Vision

A person with 20/60 vision can see at 20 feet what a person with normal vision can see at 60 feet. This is generally considered a moderate visual impairment. Individuals with 20/60 vision may need glasses or contact lenses to improve their sight.

20/100 Vision

People with 20/100 vision have significant visual difficulties. They can only see at 20 feet what a person with normal vision can see at 100 feet. People with 20/100 vision will likely need corrective measures like glasses, contact lenses, or surgery to achieve better sight.

How Are Visual Acuity Scores Categorized?

Visual acuity measurements are categorized based on the degree of vision loss. Following is a breakdown of these categories.

• Mild visual impairment (20/30 to 20/60) – This is near-normal vision. If you fall within this range, you may have a little trouble reading small print or recognizing faces from a distance. You may need to wear glasses at times or for certain activities.
• Moderate visual impairment (20/70 to 20/160) – You may find it challenging to perform daily tasks if you fall within this range. Vision correction, such as with glasses or contacts, may be needed in your day-to-day life.
• Severe visual impairment (20/200 to 20/400) – This category represents a profound decrease in vision. Those with visual impairments of 20/200 vision or less are considered "legally blind." Assistance and adaptive devices may be necessary for daily activities.
• Profound visual impairment (20/500 and less) – At this level, vision is extremely limited, and you will likely require extensive assistance to carry out daily activities.

Improve Your Vision

If you’re diagnosed with a visual acuity issue by an optometrist or ophthalmologist, eyeglasses or contact lenses may be prescribed to improve your sight. Remember to follow the recommended schedule for replacing your glasses or contacts as prescribed by your eye care professional. This can ensure you have the correct prescription and help you maintain optimal vision.

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