Snellen Chart

If you have ever had a routine eye examination to update your eyeglass or contact lens prescription, or had an in-depth physical examination, then you have likely experienced a visual acuity test. It is a simple, yet highly effective test to determine how well you see at various distances. It is the familiar eye chart test that has been used by optometrists for years.

What is a visual acuity test?

The visual acuity test is used to determine the smallest letters you can read on a standardized chart (Snellen chart) or a card held 20 feet (6 meters) away. Special charts are used when testing at distances shorter than 20 feet (6 meters). Some Snellen charts are actually video monitors showing letters or images.

How the test is performed

This test may be done in a professional health care provider’s office. You will be asked to remove your glasses or contact lenses and stand or sit 20 feet (6 meters) from the eye chart keeping both eyes open. The doctor will cover one eye with a small paddle while you read out loud the smallest line of letters you can see on the chart. Numbers, lines, or pictures are used for people who cannot read, especially children. If you are not sure of the letter, you may guess. This test is done on each eye, and one at a time. If needed, it is repeated while you wear your glasses or contacts. You may also be asked to read letters or numbers from a card held 14 inches (36 centimeters) from your face. This will test your near vision.

Why the test is performed

The visual acuity test is a routine part of an eye examination or general physical examination, particularly if there is a change in vision or a problem with vision. In children, the test is performed to screen for vision problems. Vision problems in young children can often be corrected or improved. Undetected or untreated problems may lead to permanent vision damage. There are other ways to check vision in very young children, or in people who do not know their letters or numbers.

Normal results

Visual acuity is expressed as a fraction.

  • The top number refers to the distance you stand from the chart. This is often 20 feet (6 meters).
  • The bottom number indicates the distance at which a person with normal eyesight could read the same line you correctly read.

For example, 20/20 is considered normal. 20/40 indicates that the line you correctly read at 20 feet (6 meters) away can be read by a person with normal vision from 40 feet (12 meters) away. Outside of the United States, the visual acuity is expressed as a decimal number. For example, 20/20 is 1.0, 20/40 is 0.5, 20/80 is 0.25, 20/100 is 0.2, and so on. Even if you miss one or two letters on the smallest line you can read, you are still considered to have vision equal to that line.

What abnormal results mean

Abnormal results are usually a sign that you need glasses or contacts. Or it may mean that you have an eye condition that needs further evaluation by a provider.

Why share this information?

At Treasure Coast Opticians, we believe every patient should be fully informed of all procedures performed in our office, especially if it is child or a first-time wearer of corrective lenses. A knowledgeable patient is likely to be more comfortable throughout the exam process, more confident and accurate in their responses. Nothing is more important to us than your comfort and vision.

 

References

Feder RS, Olsen TW, Prum BE Jr, et al. Comprehensive adult medical eye evaluation Preferred Practice Pattern guidelines. Ophthalmology. 2016;123(1):209-236. PMID: 26581558 www.ncbi.nlm.nih.gov/pubmed/26581558.

Rubin GS. Visual acuity and contrast sensitivity. In: Schachat AP, Sadda SVR, Hinton DR, Wilkinson CP, Wiedemann P, eds. Ryan’s Retina. 6th ed. Philadelphia, PA: Elsevier; 2018:chap 13.

Updated 2/28/2019 by: Franklin W. Lusby, MD, ophthalmologist, Lusby Vision Institute, La Jolla, CA. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.

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Treasure Coast Opticians

Treasure Coast Opticians
715 17th Street, Vero Beach, FL 32960
Call 772-569-4822
Open M-F 9 a.m. – 5:30 p.m.
Saturday by Appointment

 

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