Do You Need Prescription Eye Glasses? Ask the Ubiquitous Snellen Eye Chart

The Snellen eye chart is a constant in our lives. You first meet the chart in elementary school when the school nurse tests your vision. Next, it is part of the “rite of passage” when applying for your learners permit so you can take driver's education, and it is there when you renew your driver's license and when you visit our optometrist for your routine eye exam.

We know the Big E that resides at the top of the chart and we know it has a bunch of downstairs neighbors, and the more of them we can recognize, the better our vision. Do we know what 20/20 actually means, and what part this ubiquitous chart of letters plays in determining whether or not we need prescription eye glasses?

History of the Snellen Chart

The Snellen eye chart is named for its developer, a Dutch eye doctor named Dr. Herman Snellen who came up with it in the 1860s. Although small variations have appeared over the intervening century and a half, the Big E remains king of the hill and eleven rows of progressively smaller letters fill the rest of the chart. Sometimes it is an actual paper chart hanging on the wall twenty feet away, sometimes a projection on the wall, simulating what you would see at a twenty-foot distance.

For the most part the exam remains unchanged. You sit in a chair and our optometrist directs you to read aloud the smallest row you can, in order to determine the sharpness of your distance vision. If you can read at twenty feet away what people should be able to see at that distance, you have 20/20 vision. This is usually the fourth line from the bottom.  In most states, you need 20/40 vision to drive without prescription eye glasses or contact lenses. 20/40 means what you see at a twenty feet is what those mysterious should see people can read from forty feet away. If you can only read the E, you have 20/200 vision which is very poor visual acuity. Should this be all you can see, even with corrective lenses, you are considered legally blind.

The Phoropter and Refraction

If you need eye glasses or contact lenses, our optometrist will ask you to look through an instrument, a phoropter, and read the fourth line as he flips a series of different lenses in front of your eyes while asking "Can you see better with these........or these? He is fine-tuning the lens power you require so he can come up with your prescription. This procedure is called refraction, and determines your level of nearsightedness, farsightedness, astigmatism, or presbyopia. 

When was the last time you came in for your vision test?

If you can't remember, call either our Fairfax office (703-935-4181) or our Rockville office (301-871-0180) and we can let you know.