Visual Acuity Test and the Snellen Eye Chart

A visual acuity test is one of the most important parts of an eye exam. Visual acuity is a measure of how well you can see. A visual acuity test helps your eye health provider determine whether or not you need vision correction.

While there are visual acuity tests online or even apps that claim to tell you whether you have "20/20 vision," it's best to see your eye health provider and have your visual acuity tested. While an at-home visual acuity test might be more convenient, it's more accurate to have your provider do the test with the right tools, as well as have them look at the results and tell you what they mean.

This article will go over what visual acuity testing is. You will learn how a visual acuity test is done and how it helps your provider assess your vision. You will also learn how the Snellen Eye Chart is used to test visual acuity.

Distance Visual Acuity Test

One part of a visual acuity test shows how well you can see things that are far away rather than close up. Distance visual acuity is a measurement of your degree of vision loss.

The distance part of a visual acuity test measures the smallest object that you can recognize when it's about 20 feet away from you. Usually, you will look at an eye chart that has letters of various sizes on it that get progressively smaller. You'll be asked about how many you are able to clearly see.

Distance visual acuity is usually tested by isolating each eye. You might be given something to cover one eye or hold your hand over each eye, one at a time, as you look at the chart. Then, both eyes are measured together.

This measurement on a visual acuity test is recorded as a fraction, with 20/20 being considered normal vision.

Near Visual Acuity Test

A visual acuity test can also determine how well you see things that are near you rather than off in the distance. Near visual acuity is a measurement of how well you can see close objects.

When it is part of visual acuity testing, near visual acuity can show how well your eyes see when you're doing tasks such as reading or sewing.

Near visual acuity is usually measured with both of your eyes open. You will be asked to hold a near-point card about 16 inches from your face and read the smallest line you can see.

Intermediate Visual Acuity Test

Visual acuity testing also helps your eye health provider determine how well you see things that aren't quite near but also aren't far from you. Intermediate visual acuity is a measurement of how well you see at an arm's length distance.

During this part of a visual acuity test, you'll be asked how well you can read a chart that's arm's length from your face—about 26 inches.

Intermediate visual acuity wasn't always a typical measurement in an eye examination, but it's become more important for eye providers to assess because it's the distance at which most computer screens are set, and many people work on devices or use them frequently.

Testing Visual Acuity Using Snellen Eye Chart

A visual acuity test involves several tools. The Snellen Eye Chart is one way to measure visual acuity Snellen visual acuity was named after Dutch ophthalmologist Herman Snellen and it was developed in 1862.

The Snellen Eye Chart measures visual acuity as a fraction: the distance at which the test is performed over the distance at which the smallest optotype identified subtends an angle of 5 minutes of arc and the detail of that optotype subtends 1 minute of arc.

An optotype is a standardized letter, shape, picture, or number that is shown to the person whose visual acuity is being tested.

Using the metric system, the formula is the viewing distance (measured in meters) over the letter size (measured in M-units). For example, 6/6 is the same as 20/20.

Having 20/20 visual acuity means that a person can see small detail from 20 feet away the same as a person with normal eyesight would see it from 20 feet. If a person has a visual acuity of 20/40, they can see detail from 20 feet away the same as a person with normal eyesight would see it from 40 feet away. 

Average human visual acuity is said to be 20/20. However, many people can see 20/15 and a few even have 20/10 visual acuity. It is possible to have vision superior to 20/20—the maximum acuity of the human eye without visual aids (such as binoculars) is generally thought to be around 20/10.

Summary

Visual acuity testing is not a substitute for a comprehensive eye examination. However, it is one of the most important measurements an eye provider can perform when evaluating the human visual system. Visual acuity can be tested at a distance as well as up close, and it can help your eye health provider determine if you need vision correction.