The following basic vision tests help to assess how well your eyes are functioning and to detect potential vision disorders. They are ordinarily included as part of a full eye examination. Techniques vary depending on whether you are a new or returning patient to the eye practitioner.
Visual acuity tests help to evaluate your ability to distinguish the form and detail of an object or printed figure. This is usually done with Snellen charts, which are composed of standardized rows of different-sized letters. You are asked to read the letters on a chart from a distance of 14 to 20 feet. The smaller the letters you can identify, the better your visual acuity. The score 20/20 means you have normal vision at 20 feet.
The pinhole test helps to determine whether reduced visual acuity is due to a refractive error (an inability of the cornea and lens of the eye to bend light rays into proper focus on the retina) or to an organic vision disorder. You look through a pinhole (in the center of a disk) at the Snellen chart. If visual impairment is due to a refractive error, your vision will improve when looking through the pinhole, which eliminates irregular light rays.
The Amsler grid—a grid with a black dot at its center printed on a piece of paper—is an extremely sensitive test to detect blind spots, distortion, and other problems in central vision. You are asked to focus your eyes on the dot and note any blurring, distortion, or missing lines on the grid. Amsler grid helps to detect problems affecting the macula—the most sensitive area of the retina responsible for central vision and seeing fine detail.
Color vision tests use plates made up of dot patterns in the primary colors superimposed on backgrounds of randomly mixed colors to assess the ability to recognize color differences. People with normal color vision are able to identify the number shown by the dot pattern. If color vision is defective, you will be unable to distinguish between the dot pattern and the background. Impaired color vision results from dysfunction of specialized cells, called cones, in the retina and disorders of the optic nerve.
Purpose of the Vision Tests
- Performed as part of a regular eye examination for glasses and routine screening for vision disorders such as cataracts, glaucoma, or eye changes associated with diabetes
- To assess the eyes of people who report vision problems.
- To evaluate people with neurologic (nerve) disorders or other systemic conditions that may affect the eyes
Who Performs Vision Tests
- An ophthalmologist, an optometrist, a nurse, or an ophthalmic technician
Special Concerns about Vision Tests
Before the Vision Tests
- If you normally wear eyeglasses or contact lenses, wear them to the exam.
What You Experience during Vision Tests
Visual acuity tests:
- You will sit in a chair 20 feet from an illuminated eye chart (letters may also be projected on a reflecting screen or displayed on a TV monitor).
- You are first tested while wearing your most recently prescribed glasses or contact lenses. You will be asked to read all the letters or numbers in each progressively smaller line until you cannot discern the figures. Each eye is tested separately (while the other eye is covered).
- If visual acuity is less than 20/20, the examiner will try different lenses to learn if a change of your glasses or contact lens prescription will improve visual acuity (a process known as “refraction”). The pinhole test is often used at the beginning of refraction as a quick test.
- For this test, you will usually wear your glasses or contact lenses.
- You will be asked to cover one eye. With the other eye, you will look at the eye chart through a pinhole in the center of a disk.
- Read aloud the smallest line of letters you can see on the chart.
- The procedure is repeated for the other eye.
- You will wear your regular, bifocal, or reading glasses or contact lenses for this test.
- You will be instructed to cover one eye, and the Amsler grid is held in front of the uncovered eye.
- You are asked to look directly at the black dot at the center of the grid.
- Report any irregularities that you see on the grid, such as wavy or blurred lines, blank spots, gray areas, or other distortions in the grid pattern. You may be asked to outline (with a pencil) any areas on the grid that appear distorted.
- The procedure is repeated for the other eye.
Color vision test:
- You will wear your glasses or contact lenses for this test.
- Each eye is tested separately.
- You will be given a test book containing color plates, and asked to read aloud the number or numbers shown in each color plate.
- The number of accurate responses is recorded.
Risks and Complications
After the Vision Tests
- You may resume your normal activities.
Results of Vision Tests
- An ophthalmologist or optometrist reviews the test data for evidence of any visual defect, and considers these findings along with your eye exam and any symptoms you report.
- In cases of simple vision dysfunction, such as myopia (nearsightedness), these tests will be sufficient to prescribe glasses or contact lenses.
- If a more serious vision disorder is suspected, an in-depth eye examination and more specialized tests will be necessary to establish a diagnosis.
The Johns Hopkins Consumer Guide to Medical Tests
Simeon Margolis, M.D., Ph.D., Medical Editor
Updated by Remedy Health Media