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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 away. Special charts are used when testing at distances shorter than 20 feet.
Eye test - acuity; Vision test - acuity; Snellen test
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.
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 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.
No special preparation is necessary for this test.
There is no discomfort.
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.
Visual acuity is expressed as a fraction.
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.
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.
There are no risks.
American Academy of Ophthalmology Preferred Practice Patterns Committee. Preferred Practice Pattern Guidelines. Comprehensive Adult Medical Eye Evaluation. Available at http://one.aao.org/CE/PracticeGuidelines/PPP_Content.aspx?cid=64e9df91-dd10-4317-8142-6a87eee7f517. Accessed February 26, 2013.
Colenbrander A. Measuring vision and vision loss. In: Tasman W, Jaeger EA, eds. Duane's Ophthalmology. 2013 ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2012:vol 5, chap 51.
Miller D, Schor P, Magnante P. Optics of the normal eye. In: Yanoff M, Duker JS, eds. Ophthalmology. 3rd ed. Philadelphia, PA: Elsevier Mosby; 2008:chap 2.6.