Vision test - refraction

Alternative names
Eye test - refraction; Refraction test

Definition
The refraction test is an eye exam that measures a person’s ability to see an object at a specific distance.

How the test is performed

This test is performed by an ophthalmologist or optometrist (eye doctor). You sit in a chair that has a special device (phoropter or refractor) attached to it and look through the phoropter at an eye chart approximately 20 feet away. The phoropter contains lenses of different strengths that can be moved into view.

The eye doctor will ask if the chart appears more or less clear with the lenses that are in place.

The eye doctor can determine if you have nearsightedness, farsightedness, astigmatism (asymmetrical cornea), or presbyopia (inability to focus on objects that are close to you). The extent of vision difficulty can be determined. The information obtained from a refraction test allows the prescription for eyeglasses or contact lenses to be correct for each person.

How to prepare for the test

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

For children:
The preparation you can provide for this test depends on your child’s age, previous experiences, and level of trust. For specific information regarding how you can prepare your child, see the following topics:

     
  • 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

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 performed to help follow the progress of treatments or diseases of the eye.
The test is used to prescribe glasses if needed.

Normal Values
A normal value is 20/20 vision (perfect vision; able to read 3/8 inch letters at 20 feet)

What abnormal results mean

     
  • myopia (nearsightedness)  
  • hyperopia (farsightedness)  
  • astigmatism (asymmetrical cornea)  
  • 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

What the risks are
There are no risks.

Special considerations

A complete eye examination should be done every 3 to 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 to 3 years.

Johns Hopkins patient information

Last revised: December 3, 2012
by Gevorg A. Poghosian, Ph.D.

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