Visual acuity test

Eye — The eye is the organ of sight, a nearly spherical hollow globe filled with fluids (humors). The outer layer or tunic (sclera, or white, and cornea) is fibrous and protective. The middle tunic layer (choroid, ciliary body and the iris) is vascular. The innermost layer (the retina) is nervous or sensory. The fluids in the eye are divided by the lens into the vitreous humor (behind the lens) and the aqueous humor (in front of the lens). The lens itself is flexible and suspended by ligaments which allow it to change shape to focus light on the retina, which is composed of sensory neurons.

Also known as: Eye test - acuity, Vision test - acuity or Snellen test

Definition

The visual acuity test is used to determine the smallest letters a person can read on a standardized chart (Snellen chart) or a card held 14 - 20 feet away.

How the test is performed

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

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

Gently cover one eye with the palm of your hand, a piece of paper, or a paper cup while you read out loud the smallest line of letters you can see on the chart. Numbers 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, 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 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 Values

Visual acuity is expressed as a fraction.

The top number refers to the distance you stand from the chart. This is usually 20 feet.
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 away can be read by a person with normal vision from 40 feet away.

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 health care provider.

What the risks are

There are no risks.

References

Colenbrander A. Measuring vision and vision loss. In: Tasman W, Jaeger EA, eds. Duane's Ophthalmology. 15th ed. Philadelphia, Pa: Lippincott Williams & Wilkins; 2009:chap 51.

Miller D, Schor P, Magnante P. Optics of the normal eye. In: Yanoff M, Duker JS, eds. Ophthalmology. 3rd ed. St. Louis, MO: Mosby Elsevier;2008:chap 2.6.

American Academy of Ophthalmology Preferred Practice Patterns Committee. Preferred Practice Guidelines. Comprehensive Adult Medical Eye Evaluation. San Francisco, CA: American Academy of Ophthalmology, 2010. Accessed January 17, 2011.

Review date:: February 10, 2011
Reviewed by:: Linda J. Vorvick, MD, Medical Director, MEDEX Northwest Division of Physician Assistant Studies, University of Washington School of Medicine; and Franklin W. Lusby, MD, Ophthalmologist, Lusby Vision Institute, La Jolla, California. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.

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