Discovering Vision Therapy Blog

Why We Don’t Trust the Snellen Chart

Posted by Greg Mischio

 Snellen Chart

Say optometrist, and the first thing most people think of is the Snellen chart.  The rows of letters, descending in size, are emblematic of eye care.  Unfortunately, the Snellen chart isn’t telling you the whole story when it comes to a person’s vision.

The Snellen chart only tests for acuity.  Acuity is the clarity with which your eyes see an object.  The most familiar acuity test places the test at a standard distance of twenty feet. At the bottom of the chart is the 20/20 line. A person with 20/20 vision can accurately read this line while standing at the test distance of twenty feet. 

That’s great, but it doesn’t tell you if a person has any depth perception, or if his or her eyes can follow a line of text across a page or switch focus from far to near efficiently.  The Snellen chart is useful for detecting near-sightedness, but it does not always detect astigmatism, farsightedness, eye-teaming difficulties or even sometimes amblyopia. 

Susceptible to cheating

A surprising fact to parents is that the Snellen eye chart is susceptible to cheating. It’s not always easy for even experienced professionals to catch them.  If a child is worried about failing the test and having to get glasses, they can memorize the sequence and repeat it back when the alternate eye is tested.

It’s also not uncommon for a patient to peek with the other eye to find the right answer.

We’ve seen several adult patients over the years who report that as children they would cover the eye that didn’t see well first and then when they were supposed to switch to the other eye they would cover the same eye again so that they could read the letters.

Trust a Functional Vision Test

We trust the Snellen chart when it comes to measuring visual acuity, but we need to stress to parents, educators and health practitioners that there are other critical visual skills that require testing.

By relying too heavily on vision screening with only the Snellen eye chart, many vision problems are not being detected, which can lead to misdiagnoses of behavior problems, ADHD, and dyslexia.

It is also impossible to cheat on comprehensive Functional Vision testing.  Many visual skills can be tested objectively, so you’re not dependent on the patient’s response.  Why is this necessary?  Young children may not give accurate answers, for example, out of fear a failed test may result in glasses. 

Functional Vision Test can provide a much more accurate picture of a person’s vision.  The Vision Therapy Center encourages you to make sure you or your child receive this type of comprehensive vision testing to ensure the health of your entire visual system.

(Photo by pj_vanf)

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Topics: Snellen Eye Chart