What Exactly is an Eye Chart?

If there's one aspect of optometry that everyone recognizes, it's the traditional eye chart with its rows of big letters that gradually become smaller. This chart is usually known as the Snellen chart.

What Is an Eye Chart?

An eye chart is one of the tools your eye doctor uses to assess your eyesight. Based on how well you can see various letters on the chart, your optometrist will determine whether you have myopia (nearsightedness), hyperopia (farsightedness), presbyopia, or astigmatism, and will measure the prescription that will give you the clearest, most comfortable vision.

Are All Eye Charts The Same?

There are a number of variations to the standard Snellen eye chart. The one an eye doctor uses depends on the personal needs and abilities of the patient. For example, eye doctors will use charts with pictures or patterns for younger children who may not have learned to read, and certain charts specifically measure distance vision while others are better for measuring near vision.

History of the Snellen Eye Chart

The Snellen eye chart was developed by Dutch eye doctor Hermann Snellen in the 1860s. Before this standardized eye chart was developed, each eye doctor had their own chart they preferred to use. Having so many different charts made it impossible to standardize the vision correction available to patients. The Snellen chart for the first time allowed a person to provide a standardized prescription from any eye care provider to any eyeglass maker.

How The Snellen Chart Is Used in Eye Exams

The standard Snellen chart displays 11 rows of capital letters, with the first row consisting of a single large letter. Your eye doctor will ask you to look through a phoropter — an instrument used to test individual lenses on each eye during an eye exam — and look at the Snellen chart placed 20 feet away.

If you have 20/20 vision, it means you can see what an average person can see from a distance of 20 feet. If you have 20/200 vision (the legal definition of blindness), what a person with perfect vision can see from 200 feet away, you can only see from 20 feet away.

Does 20/20 Visual Acuity Mean Perfect Vision?

No. While eye chart tests identify refractive errors, they can't detect signs of visual skill deficiencies or diseases such as glaucoma, cataracts, or macular degeneration. These are diagnosed using advanced equipment as part of a comprehensive eye exam. Call St. Clairsville Eyecare in St. Clairsville at (740) 695-0444.