It’s a common question. As you’re sitting in the waiting room, squinting at magazines, you may wonder why exactly you don’t see everything in perfect clarity. You may often hear the phrase “20/20 vision” when your eye doctor tells you your eyes are healthy, or that you will have it after corrective lenses. It’s possible you’ve even heard about the television show named after the phrase, or even on the Discovery channel. But what does it mean?
The Big “E”
While you may think the white chart with the big “E” at the top is comical, it’s scientifically designed to determine your vision as compared to others. The United States has determined that the “average” person can see the chart when standing twenty feet away, hence the term 20/20 vision. This means that when you have 20/40 vision, you can see what the “average” human sees at twenty feet when you stand forty feet from the chart. In the United States, the legal cutoff for blindness is 20/200.
If you’re thinking that there’s no way your chair in the doctor’s office is twenty feet from the white chart, you’re probably correct. Some doctor’s offices use mirrors to simulate the twenty-foot distance, which still allows the test to be done properly.
The chart, also known as the Snellen chart, usually has each row of letters numbered. The top, the big “E” that the chart is known for, reflects 20/200 vision. While the big “E” may seem comically large to you, there are people who can see that letter from the same distance as you, but barely be able to detect it.
Likewise, the fourth line is reflective of 20/20 vision. The lines below that reflect 20/15, 20/10, and 20/5 vision, although humans rarely have 20/10 vision. Animals such as hawks are known for their acute vision—right around 20/5.
Other Options
Not everyone is able to take the Snellen vision test: if you have a disability or a shy child, you may use the “Tumbling E” chart instead. In this modification of the Snellen test, the “E” at the top of the chart is rotated in increments of ninety degrees. The person receiving this test will be instructed to demonstrate the direction the “E” is facing with their fingers by point up, down, right, or left. The size of the “E,” as well as the numbered lines, is modeled after the Snellen test.
If you’re near-sighted, your eye doctor will use the Jaeger eye chart to determine the degree of your near vision. In this test, a small hand-held card with various sizes of text is used, along with a scale ranging from J1 to J11. In this test, J2 type is equivalent to 20/20 vision.
Too Good To Be True?
If you want to test your vision without going to an eye doctor, there are several options for eye tests done at home. The internet hosts many different eye chart simulations which only require you to measure out the proper distance.
Online tests are a good way to get an idea of your vision, although they should not be considered an official exam. While the eye chart is a vital part of an eye exam, this only measures the degree of your vision acuity. This doesn’t test your peripheral vision, depth perception, eye fluid pressure, or how healthy your retinas are. An eye chart exam primarily functions to determine whether prescription eyeglasses or contact lenses are a viable option to correcting your vision impairment.