If there's one aspect of optometry that everyone recognizes, it's the traditional eye chart — with rows of big letters on top that gradually become smaller further down. This chart is usually known as the Snellen chart. But how much do you really know about it?

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, hyperopia, 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 your 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 or identify letters and numbers. There are also certain charts that 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 that they preferred to use. Having so many different eye charts made it impossible to standardize vision correction. The Snellen chart allowed a person to provide a standardized prescription from any eye care provider to any eyeglass maker, and get the same optical lenses to accurately correct their vision.

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 — and look at the Snellen chart placed 20 feet away. In many offices where 20 feet of space isn't available, you'll view the chart through a mirror, which provides the same visual experience.

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 20/40 vision, you can only see clearly from 20 feet what a person with perfect vision can see from 40 feet.

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. Early diagnosis and treatment of eye conditions are essential to ensuring long-term vision and eye health.

For more information, give us a call at (937) 228-2020 or visit us in person at Dayton Optometric Center in Kettering, Ohio today!