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Vision Tests

Why It Is Done continued...

Visual field tests may be done:

  • To check for vision loss in any area of your visual field.
  • To screen for eye diseases, such as macular degeneration and glaucoma, which cause gaps in the visual field.
  • To look for damage to the nerves of the eye following a stroke, head injury, or other condition that causes reduced blood flow to the brain.

Color vision tests may be done:

  • As part of a routine eye exam.
  • To screen for or diagnose color blindness.
  • To screen applicants for jobs in which color perception is important, such as truck driving, electronics, or the military.

How To Prepare

No special preparation is required before having vision tests. If you wear glasses or contact lenses, bring them with you to the examination since the tests cannot be properly performed without them. If you have a copy of your current eyeglass prescription, bring it with you.

Many medicines may affect the results of vision tests. Be sure to tell your health professional about all the over-the-counter and prescription medicines you take.

Talk to your health professional about any concerns you have regarding the need for vision tests, how they will be done, or what the results will mean. To help you understand the importance of these tests, fill out the medical test information form(What is a PDF document?).

How It Is Done

Visual acuity testing

Visual acuity tests are used to evaluate eyesight. Several types of visual acuity tests may be used.

  • The Snellen test checks your ability to see at distances. It uses a wall chart camera.gif that has several rows of letters. The letters on the top row are the largest; those on the bottom row are the smallest.
    • You will stand or sit 20 ft (6 m) from the chart and be asked to cover one eye and then read the smallest row of letters you can see on the chart. If you are unable to cover your eye, an eye patch will be placed over your eye.
    • Each eye is tested separately. You may be given a different chart or asked to read a row backward to make sure that you did not memorize the sequence of letters from the previous test.
    • If you wear glasses or contacts, you may be asked to repeat the test on each eye while wearing them.
    • Let your health professional know if you have trouble reading the letters on one side of the row, or if some letters disappear while you are looking at other letters. You may have a visual field problem, and visual field tests may be needed.
  • The E chart tests the vision of children and people who cannot read. The E chart camera.gif is similar to the Snellen chart in that there are several rows, but all of the rows contain only the letter E in different positions. The top row is the largest and the bottom row of Es is the smallest. You will be asked to point in the same direction as the lines of the E. Similar charts use the letter C or pictures. These charts are also available in a handheld card.
  • The Near test uses a small card (Jaeger chart) containing a few short lines or paragraphs of printed text to test your near vision. The size of the print gradually gets smaller. You will be asked to hold the card about 14 in. (36 cm) from your face and read aloud the paragraph containing the smallest print you can comfortably read. Both eyes are tested together, with and without corrective lenses. This test is routinely done after age 40, because near vision tends to decline as you age (presbyopia).

WebMD Medical Reference from Healthwise

Last Updated: June 09, 2011
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.

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