How the Snellen Eye Chart Is Used to Test Both of Your Eyes (2024)

The Snellen eye chart is the classic chart of big and little letters used by eye care professionals, in schools, and in other settings where vision testing is done.

Snellen and other well-known eye charts are used to measure visual acuity, which refers to how well you can see without glasses or contacts. The results are used to determine if you need eyewear to correct your vision. It's part of a general assessment of your overall eye health.

This article explains how the eye chart works and what some benchmark results may mean. It adds information about modified eye charts that, for example, may be used with children who don't yet read letters.

The Standard Eye Chart

The Snellen eye chart is made of letters used as "optotypes," or symbols designed for vision testing. Its name comes from Dr. Hermann Snellen, who invented the eye chart in 1862.

This standard chart consists of 11 lines of block letters, beginning with a large single letter on the top row. The number of letters on each row increases moving from top to bottom.

The size of the letters progressively decreases, allowing for more letters on each subsequent line.

Visual Acuity Testing

The Snellen eye chart is read by covering one eye and reading aloud the letters on the chart, beginning at the top and moving toward the bottom. The smallest row of letters that the patient reads accurately determines visual acuity in the uncovered eye.

The other eye also is tested, and both eyes are tested together.

The results are based on what you can see from 20 feet away compared to what someone with normal eyesight would see. It's why normal human eyesight is sometimes expressed as 20/20.

Your results are determined by the smallest line of letters you can accurately read on the chart.

How to Read Eye Chart Results

The results of your acuity exam (chart reading) will help to diagnose the quality of your eyesight. Your vision results (acuity) will be expressed as a fraction. Fractions are shown to the left of each row and identify how well you can see.

The expression of results, such as 20/40 vision or 20/25 vision, will depend on how far down you were able to read the lines on a specific chart.

Modern optometric exam rooms are equipped with electronic eye reading charts, making it easier to produce accurate readings.

Eye Chart Versions

Various types of eye charts are available besides the Snellen. Some use pictures or patterns to measure visual acuity instead of letters. Certain charts are used to measure distance vision and others measure near vision.

There are several versions of the Snellen eye chart for people, such as young children, who cannot read the letters of the alphabet.

Some alternative charts include:

• The Tumbling E test (aka Random E) uses only a capital E which gets progressively smaller and faces in various directions (up, down, left, right).
• The LEA test uses shapes (like an apple and a square) instead of letters to test vision in very young children.
• The Jaeger chart is used to evaluate near vision.
  

Summary

The standard Snellen eye chart remains the most used in clinical practice today. Snellen results are based on what you can see from 20 feet away. Other versions of this chart and alternate visual acuity charts are available. While you can access versions of the Snellen eye test chart online, it's best to have regular examinations performed by an eye care professional.