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Health Encylopedia

Visual acuity
Definition The visual acuity test measures the smallest letters that you can read on a standardized chart at a distance of 20 feet.
Alternative Names Vision Testing
How the test is performed This test may be done in a health care provider's office, a school, a work place, or elsewhere. Stand behind a line 20 feet from the eye chart. Remove glasses or contacts. Keep both eyes open and 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 that you can read on the chart. If you are not sure of the letter, you may guess. This is repeated with the other eye. Repeat the procedure while wearing glasses or contacts.
How to prepare for the test Adults: No special preparation is necessary for this test. Children: The physical and psychological preparation you can provide for this or any test or procedure depends on your child's age, interests, previous experiences, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child's age:
  • Toddler test or procedure preparation
  • (1 to 3 years)
  • Preschooler test or procedure preparation
  • (3 to 6 years)
  • Schoolage test or procedure preparation
  • (6 to 12 years)
  • Adolescent test or procedure preparation
  • (12 to 18 years)
  • How the test will feel There is no discomfort.
    Why the test is performed The test is performed when there are problems or changes in vision . In children, the test is performed to screen for any visual problems.
    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 letters at 20 feet that could be read by a person with normal vision at 40 feet.
    What abnormal results mean Abnormal results may indicate that you need corrective lenses to obtain normal vision. Related topics:
  • Nearsightedness
  • Farsightedness
  • Presbyopia
  • What the risks are
  • There are no risks.
  • Special considerations