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The refraction test is an eye exam that measures a person's prescription for eyeglasses or contact lenses.

This test is performed by an ophthalmologist or optometrist, or "eye doctor."

You sit in a chair that has a special device (called a phoroptor or refractor) attached to it. You look through the device and focus on an eye chart about 20 feet away. The device contains lenses of different strengths that can be moved into your view.

The eye doctor performing the test will ask if the chart appears more or less clear when different lenses are in place.

If you wear contact lenses, ask the doctor how long they should be left out before the test.

There is no discomfort.

This test can be done as part of a routine eye test to determine if a person has normal vision.

When a person complains of blurred vision, this test can help determine the extent of poor vision. It can also be done to monitor a person who is being treated for an eye disease.

The test is used to prescribe glasses, if needed. It also will determine if you need bifocals.

A normal value is 20/20 vision (perfect vision - able to read 3/8 inch letters at 20 feet). A small type size is also used to determine normal near vision.

Abnormal results may be due to:

• Astigmatism
• Hyperopia (farsightedness)
• Myopia (nearsightedness)
• Presbyopia (inability to focus on near objects that develops with age)

Additional conditions under which the test may be performed:

• Corneal ulcers and infections
• Macular degeneration
• Retinal detachment
• Retinal vessel occlusion
• Retinitis pigmentosa

There are no risks.

A complete eye examination should be done every 3 - 5 years if there are no problems. If vision becomes blurry, worsens, or if there are other noticeable changes, an eye examination should be scheduled immediately.

After age 40 (or for people with a family history of glaucoma), eye examinations should be scheduled more frequently to test for glaucoma. Anyone with diabetes should have an eye exam at least once a year.

People with refraction problems should have an eye examination every 2 - 3 years.

Eye test - refraction; Vision test - refraction

Yanoff M, Duker JS, Augsburger JJ, et al. Ophthalmology. 2nd ed. St. Louis, Mo: Mosby; 2004:71-77.

Behrman RE. Nelson Textbook of Pediatrics. 17th ed. Philadelphia, Pa: WB Saunders; 2004; 2084-2085.

Updated by: Paul B. Griggs, MD, Department of Ophthalmology, Virginia Mason Medical Center, Seattle, WA . Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.