Eye Exam 101: A Quick Look at Eye Charts

In the 1800s, Dutch ophthalmologist Herman Snellen had the vision (pun intended) to create an eye chart to help doctors quickly measure the visual acuity—the clarity of vision, or the measure of how well a person sees at a distance—of their patients. And you know what? In 1862 he did just that, creating a standard that doctor’s use, even today!

In fact, you’ve probably seen a Snellen chart many times before. It consists of eleven lines of letters—usually beginning with a large letter “E” at the top—that get progressively smaller the farther down you go. And despite looking like some random sampling of the alphabet, these letters (C, D, E, F, L, O, P, T and Z) actually serve a very specific purpose. Carefully selected by Snellen because of their similar shapes, these letters become almost indistinguishable from one another at a distance, and serve as an excellent way to test one’s vision. 

During an eye exam, your doctor will probably ask you to read out loud the smallest line of letters that you can make out. If you can read the bottom row perfectly, your visual acuity is very good. But if you can only make it as far as the large letter “E” at the top, then your visual acuity is pretty poor (and you’ll actually be considered “legally blind”). A Snellen chart won’t help measure peripheral vision or detect eye diseases, like glaucoma, but it will help your doctor determine whether you need prescription eyeglasses or contact lenses for your distance vision.

Ophthalmology has advanced leaps and bounds since Snellen’s day, but eye doctors all over the world still use his chart to quickly assess a person’s eyesight. So keep an eye out the next time you have an eye exam, and thank Snellen for the letters. Find a VSP eye doctor with a Snellen chart near you.