Different functions of the eye are checked by vision tests. A complete assessment of a person's eyesight consists of the tests that measure the ability to focus on and see details at near and far distances (visual acuity), check for gaps or defects in the field of vision, and evaluate the ability to see different colors. Also more specialized tests can be done, it depends on the person's symptoms, age, and medical and family history.

Visual acuity tests

Visual acuity tests are the simplest tests that are used to evaluate a person's eyesight. They measure the eye's focusing power and the person'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. Both eyes are always tested separately with and without corrective lenses (if the person wears them), and separate scores are recorded for each eye.

The visual acuity exam may also include a refraction test, which measures the eye's refractive error. A refractive error, such as nearsightedness or farsightedness, occurs when light rays entering the eye do not focus exactly on the nerve layer (retina) at the back of the eye. This leads to blurred vision. Refraction testing 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 a person's eyesight is below normal and can be corrected by glasses.

Visual field tests

A person's complete visual field is the entire area seen when the person's gaze is fixed in one direction. The complete visual field is seen by both eyes simultaneously, and it includes the central visual field and side (peripheral) visual fields.

Visual field tests can help discover eye diseases or nervous system problems that limit a person's ability to see objects clearly in the entire visual field or in one part of it. Several tests are usually done to evaluate a person's visual field.

• Confrontation test. A simple screening method that allows the examiner to estimate the person's visual field.
• Amsler grid test. A simple method to screen for macular degeneration, a disease that causes the person to lose vision in the central area of the visual field.
• Perimetry. Records the total area of the entire visual field when a person sit in front of a machine that flashes lights randomly at various points in the visual field.
• Tangent screen. Records the central visual field from a fixed point on a black screen.

Color vision tests

Color vision testing checks a person's ability to distinguish colors and is often part of a routine eye exam. It is also used to screen for color blindness in people with suspected retinal or optic nerve disease or a family history of color blindness, which is an inherited trait that affects men far more often than women. The color vision test is also used to screen applicants for jobs in fields where color perception is essential, such as the military or electronics.

Vision Tests
Results

Visual acuity testing

When an eye chart is used to test visual acuity, the visual acuity score compares the person's performance with that of someone who has "normal" vision. Each eye's score is expressed as a ratio, such as 20/20 (6/6) or 20/100 (6/30). The top number is the distance the person stands from the chart, usually 20 feet (about 6 meters) when using a typical wall chart. The bottom number is the distance from which a person with good eyesight can read the smallest line on the eye chart. For instance, a person with 20/60 vision sees at 20 feet (6 meters) what a person with normal vision can see at 60 feet (18 meters).

• 20/20 (6/6) vision is considered normal vision. A person with 20/20 vision can see at 20 feet (6 meters) what the average person 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 feet (6 meters) away what a person with normal vision can see from 10 feet (3 meters) away.
• 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 when wearing corrective lenses is considered legally blind.

For near vision, 14/14 (35/35) is the normal ratio, with 14 inches (35 centimeters, 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 inches (35 cm) away to read print that a person with normal near vision can read from 20 inches (50 cm) away.

Visual field test

Normally, a person's visual field forms a rough circle with a natural blind spot somewhere near the middle. 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.

If there are certain areas of the visual field where you were not able to see the test objects (in confrontation or tangent screen testing) or light flashes (in perimetry testing), you may have a vision loss in that area.

Abnormal results on an Amsler grid test include:

• Not being able to see the black dot at the center of the grid.
• Blank spots or dark spots on the grid (other than the black dot at the center).
• Some lines appearing wavy or curved.
• Not being able to see all four edges of the grid.

These results are signs of possible central vision loss, which can be caused by macular degeneration, advanced glaucoma, or stroke.

Defects in different parts of the visual field may arise from many causes, including diseases of the eye (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 may need further tests to determine the cause.

Color vision test

People who have normal color vision are able to distinguish the colored numbers or symbols 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, depending on what type of color vision problem you have.