Vision Tests

Vision tests check many different functions of the eye. The tests measure your ability to see details at near and far distances, check for gaps or defects in your field of vision, and evaluate your ability to see different colors.

• Visual acuity tests are the most common tests used to evaluate eyesight. They measure the eye's ability to see details at near and far distances. The tests usually involve reading letters or looking at symbols of different sizes on an eye chart. Usually, each eye is tested individually and then both eyes are tested together with and without corrective lenses (if you wear them). Several types of visual acuity tests may be used.
• Refraction is a test that measures the eyes' need for corrective lenses (refractive error). It is usually done after a visual acuity test. Refractive errors, such as nearsightedness or farsightedness, occur when light rays entering the eye do not focus exactly on the nerve layer (retina) at the back of the eye. This causes blurred vision. Refraction is done as a routine part of an eye examination for people who already wear glasses or contact lenses, but it will also be done if the results of the other visual acuity tests show that your eyesight is below normal and can be corrected by glasses.
• Visual field tests are used to check for gaps in your side (peripheral) vision. Your complete visual field is the entire area seen when your gaze is fixed in one direction. The complete visual field is seen by both eyes at the same time, and it includes the central visual field—which detects the highest degree of detail—and the peripheral visual fields.
• Color vision tests check your ability to distinguish colors. It is used to screen for color blindness in people with suspected retinal or optic nerve disease or who have a family history of color blindness. The color vision test is also used to screen applicants for jobs in fields where color perception is essential, such as law enforcement, the military, or electronics. Color vision tests only detect a problem; further testing is needed to identify what is causing the problem.

Visual acuity tests may be done:

• As part of a routine eye exam to screen for vision problems. How often you should have routine eye exams changes as you age:
  • Once between the ages of 3 and 5
  • Before a child begins first grade, then every 2 years
  • Once between the ages of 20 and 29
  • Twice between the ages of 30 and 39
  • Every 2 to 4 years between the ages of 40 and 65
  • Every 1 to 2 years after age 65
• To monitor an eye problem, such as diabetic retinopathy.
• To determine if you need glasses or contact lenses to improve your vision.
• After an injury to the eye.
• When you obtain or renew your driver's license or for some types of employment.

Refraction is done:

• To determine the correct prescription for eyeglasses or contact lenses.

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 examination.
• 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.

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 indicate. To help you understand the importance of these tests, fill out the medical test information form (What is a PDF document?).

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 that has several rows of letters. The letters on the top row are the largest; those on the bottom row are the smallest. See a picture of a Snellen chart .
  • 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 and then together. You may be given a different chart or asked to read a row backward to make sure that you did not remember 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 doctor 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 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. See a picture of an E chart .
• The Near test uses a small card containing a few short lines or paragraphs of printed matter 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. As with the Snellen test, each eye is tested separately and then together, with and without corrective lenses. This test is routinely done after age 40, because near vision tends to decline as you age (presbyopia).

If you cannot read any of the letters or print on these charts because of poor vision, your visual acuity will be tested by other techniques, such as counting fingers, detecting hand movements, or distinguishing the direction or perception of light sources (such as room light or a penlight held up close to the face).

Visual acuity tests usually take about 5 to 10 minutes. They may be performed by a nurse, a medical assistant, an ophthalmologist, an optometrist, a teacher, or some other trained person. Testing may be done at a doctor's office, school, workplace, health fair, or elsewhere.

Refraction

Refraction is a test that measures the eye's need for a corrective lens (refractive error). For this test, you will be asked to describe the effects of looking at an eye chart through various corrective lenses.

A series of trial lenses is placed in front of your eyes and adjusted until the light rays are properly focused on your retina. Testing one eye at a time, the health professional will ask you to compare the effects of two lenses (first one lens, then the other). You should state which lens of each pair gives you better vision. The health professional will continue to test your eyes with different lenses until it is determined which lenses correct your vision the best.

Visual field tests

Visual field tests are used to check for gaps in your range of vision. They can help detect eye diseases or nervous system problems that limit your ability to see objects clearly in the entire visual field or in one part of it. Several tests are commonly done to evaluate a person's visual field.

• The confrontation test. Your health professional will sit or stand 2 ft (0.6 m) to 3 ft (1 m) in front of you. You cover one eye while fixing your gaze on the health professional's nose. The health professional slowly moves a finger or hand from the outer edge of your visual field toward the center and from the center toward the edge through all areas of your visual field. You will focus your eye on your doctor's nose and signal when you first see the health professional's finger or hand. The test is then repeated for the other eye.
• The Amsler grid test checks for macular degeneration, a disease that causes loss of vision in the center of your visual field. The test uses a 4 in. (10 cm) square chart with straight lines that form boxes. The grid has a black dot at the center. The chart is held about 14 in. (36 cm) from your face. You will cover one eye while focusing your other eye on the black dot. The test is then repeated on the other eye. Tell your doctor if:
  • You cannot see the black dot.
  • You see a blank or dark spot (other than the center dot).
  • The lines in the grid look wavy, blurred, or curved instead of straight. You will be asked to point to the specific abnormal area of the grid.
• Perimetry testing uses a machine that flashes lights randomly at various points in the visual field. You look inside a bowl-shaped instrument called a perimeter. While you stare at the center, lights will flash, and you press a button each time you see a flash. A computer records the location of each flash and whether you pressed the button when the light flashed in that location. At the end of the test, a printout shows any areas of your visual field where you did not see the flashes of light. In an alternative manual perimetry test, your health professional moves a light target and notes your visual field on paper.
• The tangent screen test uses a black screen with concentric circles and lines leading out from a center point (like a bull's-eye). Sitting 3 ft (1 m) to 6 ft (2 m) away from the screen, you cover one eye while fixing your gaze on a target point marked on the screen. Test objects of various sizes at the tip of a wand are then moved inward from the outer edge of the screen toward the center. You will signal when you can see the object, and that point is then marked on the screen. The points on the screen where you see the objects are connected to provide an outline of your visual field. The test is then repeated for the other eye. An alternative manual tangent screen test uses a white object against a black background. If you wear glasses, you will keep them on for this test.

Color vision test

Color vision tests check your ability to distinguish colors. In the most commonly used color vision test, you look for different colored numbers or symbols hidden in varying backgrounds of colored dots.

First, you are shown sample patterns and told what symbols and numbers you can expect to see. You then sit at a table and cover one eye. The health professional holds the color test patterns about 14 in. (36 cm) away from you. Some patterns are more difficult to pick out than others. As the health professional holds up a pattern, you will identify the number or symbol you see and trace it using a pointer. Some patterns may not have a number or symbol. The test is then repeated with the other eye.

You should not feel any discomfort during these vision tests.

There are no risks associated with vision tests.

Vision tests check many different functions of the eye.

Visual acuity testing

The visual acuity score compares your performance with that of people who have normal vision, using an eye chart. Each eye's score is expressed as two numbers, such as 20/20 (6/6) or 20/100 (6/30). The first number is the distance you stand from the chart, usually 20 ft (6 m) when using a typical wall chart. The second number is the distance from which people with normal eyesight can read the same line on the eye chart.

20/20 (6/6) vision is considered normal. A person with 20/20 vision can see at 20 ft (6 m) what people with normal vision can see at this distance.

• When the second number is smaller than the first number, the person's vision is better than normal. For instance, a person with 20/10 (6/3) vision can see from 20 ft (6 m) what people with normal vision can see from 10 ft (3 m).
• When the second number is larger than the first number, the person's vision is worse than normal.
• A person with 20/200 (6/60) vision or less in his or her best eye when wearing corrective lenses is considered legally blind.

For near vision, 14/14 (35/35) is normal, with 14 in. (36 cm) being the normal distance for reading. If the bottom number is greater than 14 (14/20, for example, or 35/50), it means that you have reduced near vision. You have to be 14 in. (36 cm) away to read print that people with normal near vision can read from 20 in. (51 cm).

Visual acuity tests usually take about 5 to 10 minutes.

Refraction

The health professional tests your eyes with different lenses until the lens that corrects your vision the best (sometimes better than 20/20 or 6/6) is found. The result of a refraction test determines your prescription eyeglass or contact lens strength.

A refraction test takes 10 to 30 minutes (30 minutes if dilating drops are used).

Visual field test

Normally, a person's visual field forms a rough circle with a natural blind spot. If your vision is normal, you should be able to see objects clearly throughout the entire visual field except for the area with the natural blind spot. When you are using both eyes to see, the blind spots do not interfere with your vision.

You may have vision loss in certain areas of the visual field if you are not able to see:

• Test objects during tangent screen testing.
• Movements or light flashes during perimetry testing.

Abnormal results during Amsler grid testing include:

• Not being able to see the black dot at the center of the grid.
• Not being able to see all four edges of the grid.
• Having blank spots or dark spots on the grid (other than the black dot at the center).
• Seeing lines that look wavy or curved.

Gaps in different parts of the visual field may have many causes, including eye diseases (such as glaucoma and macular degeneration) or nervous system problems (such as stroke). If results on any of the visual field tests are abnormal, you will need further tests to determine the cause.

Confrontation tests and Amsler grid tests take just a few minutes. More thorough visual field testing that uses perimetry and tangent screens can take more than 45 minutes when both eyes are tested.

Color vision test

People who have normal color vision are able to distinguish the colored numbers, symbols, or paths from the background of colored dots.

If you are not able to distinguish some or all of the colored patterns from the background, you may have a color vision problem. You may be able to pick out some patterns of colors but not others, or you may be able to pick out patterns that are different from a person with normal vision, depending on what type of color vision problem you have.

This test takes only a few minutes.

Many conditions can change your vision test results. Your health professional will discuss any significant abnormal results with you in relation to your symptoms and medical history.

Factors that can interfere with your test or the accuracy of the results include:

• Your ability to understand or follow instructions. Some vision tests cannot be done on babies, small children, or people who cannot understand or follow the instructions.
• Your ability to stay alert and respond to questions.
• Failure to wear prescribed eyeglasses or contact lenses.
• Poor lighting.

• Eye charts that use pictures or symbols (such as an E chart) may be needed to test children or people who cannot read. Vision tests are also available for babies and young children.
• Routine vision testing is usually a part of every general physical examination during childhood. Schedule routine vision checks and eye examinations with an ophthalmologist or optometrist. For more information, see the topic Nearsightedness (Myopia).
  • Eye exams for babies
  • Eye exams for children and teenagers
  • Eye exams for adults
• A complete eye and vision evaluation also includes a physical examination of the structures inside the eye. For more information, see the medical test Ophthalmoscopy.
• A test to screen for increased intraocular pressure (IOP), which increases your risk for glaucoma, is often part of a routine eye examination. It also is used to monitor treatment for glaucoma. Tonometry can be used to determine whether a medicine is keeping your IOP below a set target pressure determined by your doctor. For more information, see the medical test Tonometry.
• Home tests for near vision in adults and distance vision in children are available. These tests should not replace a thorough eye examination by a health professional.

Other Works Consulted

• Chernecky CC, Berger BJ, eds. (2004). Laboratory Tests and Diagnostic Procedures, 4th ed. Philadelphia: Saunders.

• Fischbach FT, Dunning MB III, eds. (2004). Manual of Laboratory and Diagnostic Tests, 7th ed. Philadelphia: Lippincott Williams and Wilkins.

• Handbook of Diagnostic Tests (2003). 3rd ed. Philadelphia: Lippincott Williams and Wilkins.

• Pagana KD, Pagana TJ (2002). Mosby’s Manual of Diagnostic and Laboratory Tests, 2nd ed. St. Louis: Mosby.