Visual acuity refers to the sharpness of your vision at a certain distance (typically 20 feet). Visual acuity is expressed as a fraction. When people say they have 20/20 vision, they’re talking about their visual acuity.
Visual acuity is one factor that contributes to your overall vision. Others include:
- Depth perception
- Color vision
- Peripheral awareness (side vision)
- Eye coordination
Most of the time, people with low visual acuity must wear corrective lenses like glasses or contact lenses. Refractive eye surgery, such as LASIK, can also help.
What Is 20/20 Vision?
Having 20/20 vision means you can see the same level of detail from a distance of 20 feet as an average person.
20/25 vision means that you need to be 20 feet away to see clearly what a person with normal eyesight can see from 25 feet away.
What Contributes to Visual Acuity?
Visual acuity is dependent on a combination of optical and neural elements. These include:
- The health and functionality of the retina
- How sharply the retina forms images
- How well your brain can interpret sensory inputs
What Is a Visual Acuity Test?
A visual acuity test is a type of eye examination that measures your ability to see details at a specific distance.
Optometrists use visual acuity tests to help determine the level of vision correction required for your eyeglasses or contact lens prescription.
Visual acuity is measured by charts, optical instruments, or computerized tests during an eye exam. Visual acuity tests are noninvasive and don’t involve any risks or special preparation.
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What Are the Types of Visual Acuity Tests?
Eye charts are the most common type of visual acuity test. They show several rows of optotypes (letters, numbers, or symbols) progressively smaller towards the bottom.
The most frequently used visual acuity tests are the Snellen Eye Chart (left) and the Random E Chart (right).
Snellen Test
The Snellen test is the visual acuity test most people are familiar with. It uses the Snellen chart, which contains many rows of different letters that become increasingly smaller. The letters on a Snellen chart are large at the top, with the smallest line at the bottom.
Your eye care provider will ask you to read lines on the Snellen chart until you can’t make the letters out anymore. Some Snellen charts are projected onto the wall.
Random E Test
The random E test is similar to the Snellen test but only uses the capital letter E. The letters on the random E chart are flipped (rotated) in various directions. Like the Snellen test, the random E test has letters that get smaller toward the bottom.
Your eye care specialist will ask you to read lines on the random E chart until you can’t tell which way the letter E is facing.
Visual Acuity Tests for Children
Because young children may not yet be able to name letters, they have special visual acuity tests.
A pediatric eye care specialist may have an eye chart containing symbols children can easily identify.
What Does a Visual Acuity Score Mean?
A visual acuity score results from a visual acuity test performed by an optometrist or ophthalmologist during an eye exam.
Visual acuity scores are usually written as a fraction. These test results describe your ability to see an object 20 feet away compared to a person with normal vision.
For example, 20/20 vision means seeing an object clearly from 20 feet away. The medical term for this is emmetropia.
What Vision Problems Affect Visual Acuity Scores?
Various eye conditions can affect visual acuity, including refractive errors and neural factors. A comprehensive eye exam by an eye care specialist can help diagnose these problems.
Refractive Errors
Refractive errors or neural factors usually cause poor visual acuity. A refractive error is a visual impairment that affects the way light is bent, or refracted, in your eyeball.
Common refractive errors include:
- Hyperopia (farsightedness). This is blurry near vision but clear distance vision.
- Myopia (nearsightedness). This is blurry distance vision while close objects are clear.
- Presbyopia (age-related farsightedness). A loss of focusing ability on near objects that occurs with age.
- Astigmatism. Irregular curvature of the eyeball that causes blurred vision for objects up close and far away.
Neural Factors
Neural factors occur in the retina, brain, or the pathway between the two. Examples include:
- Age-related macular degeneration (AMD)
- Detached retina
- Amblyopia (lazy eye)
- Brain injuries
- Stroke
What Is the Purpose of a Visual Acuity Test?
A visual acuity test will help assess your visual function and diagnose common vision problems, such as refractive errors. It’s a key part of a comprehensive eye exam.
Children frequently get their visual acuity tested to detect potential vision problems early on, while they’re easiest to treat.
You will likely need to take a corrected visual acuity test (wearing your glasses or contacts, if you use them) to get a driver’s license.
How Is a Visual Acuity Test Performed?
An optometrist or ophthalmologist typically performs a visual acuity test as part of a comprehensive eye exam. Opticians, technicians, and nurses can also perform a visual acuity test.
You’ll be seated a certain distance from the visual acuity chart during the eye exam. Elements such as distance from the test chart and lighting conditions must be standardized.
You’ll cover one eye and read the chart with the uncovered eye. You’ll start at the top and proceed downwards until you can’t distinguish the letters.
Then you’ll switch to the other eye and repeat the process from the same viewing distance. The doctor will use the smallest visible letter size line to determine your results.
Visual Acuity Test Interpretation
Visual acuity is typically measured as a fraction or decimal number. The first is the testing distance, and the second refers to the distance from which the average person could see the same details. The higher the second fraction number, the lower the visual acuity.
Most vision testing in the United States uses the Snellen chart, which requires a test distance of 20 feet. In other countries, visual acuity is expressed using the metric system. Because 20 feet is equivalent to 6.096 meters, 20/20 vision is equal to 6/6 vision.
For example, 6/12 is a metric measurement on the Snellen test. It’s equivalent to 20/40 in feet, .50 in decimal, and .30 on the LogMAR scale.
20/20 vision means that a person can see an object, as does anyone with normal vision from 20 feet.
LogMAR Chart
The National Vision Research Institute of Australia developed the LogMAR chart (Logarithm of the Minimum Angle of Resolution) in 1976.
The Bailey-Lovie chart and ETDRS chart (Early Treatment Diabetic Retinopathy Study) are two charts that use the LogMAR scale.
Its design provides a more accurate visual acuity score than other charts. With a LogMAR chart, your visual acuity score is measured in reference to the minimum angle of resolution.
Visual Acuity Scale
| Feet | Meters | Decimals | LogMAR |
| 20/200 | 6/60 | 0.10 | 1.00 |
| 20/160 | 6/48 | 0.125 | 0.90 |
| 20/125 | 6/38 | 0.16 | 0.80 |
| 20/100 | 6/30 | 0.20 | 0.70 |
| 20/80 | 6/24 | 0.25 | 0.60 |
| 20/63 | 6/19 | 0.32 | 0.50 |
| 20/50 | 6/15 | 0.40 | 0.40 |
| 20/40 | 6/12 | 0.50 | 0.30 |
| 20/32 | 6/9.5 | 0.63 | 0.20 |
| 20/25 | 6/7.5 | 0.80 | 0.10 |
| 20/20 | 6/6 | 1.00 | 0.00 |
| 20/16 | 6/4.8 | 1.25 | −0.10 |
| 20/12.5 | 6/3.8 | 1.60 | −0.20 |
| 20/10 | 6/3 | 2.00 | −0.30 |
Summary
Visual acuity refers to your ability to see details at a specific distance. Most of the time, visual acuity is expressed as a fraction, such as 20/20 vision.
An eye care specialist will perform a visual acuity test during a routine eye exam. This helps detect vision problems and determine the level of vision correction needed for your eyeglass prescription.
The most common types of visual acuity tests use the Snellen chart or the random E chart.
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