Skip to main content

Health Encyclopedia

Search the Health Encyclopedia

Visual acuity test

Definition

The visual acuity test is used to determine the smallest letters you can read on a standardized chart (Snellen chart) or a card held 20 feet (6 meters) away. Special charts are used when testing at distances shorter than 20 feet (6 meters). Some Snellen charts are actually video monitors showing letters or images.

Alternative Names

Eye test - acuity; Vision test - acuity; Snellen test

How the Test is Performed

This test may be done in a health care provider's office, a school, workplace, or elsewhere.

You will be asked to remove your glasses or contact lenses and stand or sit 20 feet (6 meters) from the eye chart. You will keep both eyes open.

You will be asked to cover one eye with the palm of your hand, a piece of paper, or a small paddle while you read out loud the smallest line of letters you can see on the chart. Numbers, lines, or pictures are used for people who cannot read, especially children.

If you are not sure of the letter, you may guess. This test is done on each eye, and one at a time. If needed, it is repeated while you wear your glasses or contacts. You may also be asked to read letters or numbers from a card held 14 inches (36 centimeters) from your face. This will test your near vision.

How to Prepare for the Test

No special preparation is necessary for this test.

How the Test will Feel

There is no discomfort.

Why the Test is Performed

The visual acuity test is a routine part of an eye examination or general physical examination, particularly if there is a change in vision or a problem with vision.

In children, the test is performed to screen for vision problems. Vision problems in young children can often be corrected or improved. Undetected or untreated problems may lead to permanent vision damage.

There are other ways to check vision in very young children, or in people who do not know their letters or numbers.

Normal Results

Visual acuity is expressed as a fraction.

  • The top number refers to the distance you stand from the chart. This is often 20 feet (6 meters).
  • The bottom number indicates the distance at which a person with normal eyesight could read the same line you correctly read.

For example, 20/20 is considered normal. 20/40 indicates that the line you correctly read at 20 feet (6 meters) away can be read by a person with normal vision from 40 feet (12 meters) away. Outside of the United States, the visual acuity is expressed as a decimal number. For example, 20/20 is 1.0, 20/40 is 0.5, 20/80 is 0.25, 20/100 is 0.2, and so on.

Even if you miss one or two letters on the smallest line you can read, you are still considered to have vision equal to that line.

What Abnormal Results Mean

Abnormal results may be a sign that you need glasses or contacts. Or it may mean that you have an eye condition that needs further evaluation by a provider.

Risks

There are no risks with this test.

References

Elliott DB, Flanagan JG. Assessment of visual function. In: Elliott DB, ed. Clinical Procedures in Primary Eye Care. 4th ed. Philadelphia, PA: Elsevier Saunders; 2014:chap 3.

Feder RS, Olsen TW, Prum BE Jr, et al. Comprehensive adult medical eye evaluation preferred practice pattern guidelines. Ophthalmology. 2016;123(1):209-236. PMID: 26581558 www.ncbi.nlm.nih.gov/pubmed/26581558.

Review Date:2/7/2017
Reviewed By:Franklin W. Lusby, MD, ophthalmologist, Lusby Vision Institute, La Jolla, CA. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.

The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition. A licensed medical professional should be consulted for diagnosis and treatment of any and all medical conditions. Call 911 for all medical emergencies. Links to other sites are provided for information only -- they do not constitute endorsements of those other sites. © 1997-A.D.A.M., Inc. Any duplication or distribution of the information contained herein is strictly prohibited.

adam.com

The Agency for Health Care Administration (Agency) and this website do not claim the information on, or referred to by, this site is error free. This site may include links to websites of other government agencies or private groups. Our Agency and this website do not control such sites and are not responsible for their content. Reference to or links to any other group, product, service, or information does not mean our Agency or this website approves of that group, product, service, or information.

Additionally, while health information provided through this website may be a valuable resource for the public, it is not designed to offer medical advice. Talk with your doctor about medical care questions you may have.

Health
Outcome Data

No data available for this condition/procedure.

Health Encyclopedia

More Features

We Appreciate Your Feedback!
1. Did you find this information useful?
         Yes
         No
2. Would you recommend this website to family and friends?
         Yes
         No