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 colours. 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 examinations 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 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.

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.

Colour vision tests.

These tests check your ability to distinguish colours. They are used to screen for colour blindness in people with suspected retinal or optic nerve disease or who have a family history of colour blindness. Colour vision tests are also used to screen applicants for jobs in fields where colour perception is essential, such as law enforcement, the military, or electronics. Colour 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 examination to screen for vision problems. How often you should have routine eye examinations changes as you age. Adults, children, and teens have different schedules for eye examinations.
• 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 licence 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.

Colour vision tests

These tests may be done:

• As part of a routine eye examination.
• To screen for or diagnose colour blindness.
• To screen applicants for jobs in which colour 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 examination 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 co-operate 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 he or she moves a finger into and out of your field of vision.

Colour vision test

• You look at pieces of printed test patterns in various colours. 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. He or she 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 (6/6) 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 his or her 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 centre 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 centre).
• 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, he or she is considered legally blind.

Colour vision test

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

If you are not able to distinguish some or all of the coloured patterns from the background, you may have a colour vision problem. You may be able to pick out some patterns of colours 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 colour 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.