Vision Tests

Test Overview

Vision tests check many different functions of the eye. Some of 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. Others may check how sensitive you are to glare (brightness acuity), how well your eyes work together to provide depth perception, and more. Vision tests are usually done along with exams and tests that check the health of the eye. Here are some common tests that check for blurred or low vision.

Visual acuity (sharpness) tests.

These tests help your doctor find out if you have a problem that affects how well you can see. 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 by itself. And then both eyes may be tested together, with and without corrective lenses (if you wear them). Several types of visual acuity tests may be used.

Refraction test.

This test shows your level of refractive error and finds out the right prescription for glasses or contact lenses. Refractive errors, such as nearsightedness or farsightedness, occur when light rays entering the eye can't 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 exam 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.

These 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. It includes the central visual field—which detects the highest degree of detail—and the peripheral visual fields.

Color vision tests.

These tests check your ability to distinguish colors. They are used to screen for color blindness in people with suspected retinal or optic nerve disease or who have a family history of color blindness. Color vision tests are 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. More testing is needed to identify what is causing the problem.

Why It Is Done

Visual acuity tests

These 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. Adults, children, and teens have different schedules for eye exams.
• To monitor an eye problem, such as diabetic retinopathy, or to find out if a treatment is working.
• To find out if you need glasses or contact lenses to improve your vision.
• After an injury to the eye, to check if your sight was affected.
• When you obtain or renew your driver's license or for some types of employment.
• To check the near vision of school-age children who have trouble reading, poor school performance, or blurred vision while doing work up close.

Refraction

This test is done:

• To determine the correct prescription for eyeglasses or contact lenses.
• To find out if blurred vision is caused by refractive error or eye disease.

Visual field tests

These 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

These 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

If you wear glasses or contact lenses, bring them with you to the exam since the tests cannot be properly performed without them. If you have a copy of your current eyeglass prescription, bring it with you.

If you have a young child who is being tested, it's best to practice eye tests at home before you take your child to the appointment. This can help your child cooperate better during the real testing.

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

How It Is Done

Visual acuity test

• You cover one eye at a time.
• You read aloud from a wall chart across the room.
• You read aloud from a small card that you hold in your hand.

Refraction

• You look into a special device.
• The device puts lenses of different strengths in front of each eye to see how strong your glasses or contact lenses need to be.

Visual field tests

• Your doctor may have you look through special machines.
• Or your doctor may simply have you stare straight ahead while they move a finger into and out of your field of vision.

Color vision test

• You look at pieces of printed test patterns in various colors. You say what number or symbol you see.
• Your doctor may have you trace the number or symbol using a pointer.

How It Feels

There is very little chance of having a problem from this test. If dilating drops are used for a vision test, they may make the eyes sting and cause a medicine taste in the mouth.

Risks

In some people, the dilating eyedrops can cause an allergic reaction.

Results

Vision tests check many different functions of the eye. Your doctor will let you know if your eyesight is normal or if it is better or worse than normal. The doctor may also be able to tell you why you have a vision problem.

Visual acuity testing

The visual acuity score compares your distance vision 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 distance vision is worse than normal.
• A person with 20/200 (6/60) vision or less in their best eye when wearing corrective lenses is considered legally blind.

Your doctor will also tell you if you have reduced near vision.

Refraction

The doctor 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.

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.

If visual field tests show that a person has a visual field that is 20 degrees or less, even with the help of glasses or contact lenses, that person is considered legally blind.

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.

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