You’ve heard that hindsight is 20/20, but what exactly does that mean? The term 20/20 vision is a measurement of how well you see in comparison to other people with healthy vision.
One of the most common tests administered by your optometrist was actually developed by Dutch eye doctor Hermann Snellan. The Snellan eye chart displays 11 rows of capital letters, with each row becoming progressively smaller.
During an exam, your eye doctor will instruct you to read the smallest line that you can discern. The ability to read smaller letters indicates greater visual acuity.
A standard vision chart is placed 20 feet away from the patient. However, if space does not permit, the eye chart may actually be placed behind the patient, using mirrors to create the simulation of 20 feet of distance.
The term 20/20 vision, which actually represents visual acuity, simply means that you can read what a person with normal vision can read at 20 feet away. Although the Snellan chart has a few variations, there is generally a large letter E at the top. If you can read this letter but cannot read any of the letters below it, your visual acuity is 20/200. This means that what someone with normal vision can see at 200 feet must be 20 feet away in order for you to see it clearly. This is a marker of poor visual acuity.
In fact, with corrective measures, such as contact lenses or glasses, if you still cannot make out any of the letters below the large E, you are considered legally blind. In order to get a driver’s license, your corrected visual acuity must be at least 20/40.
The fourth line up from the bottom represents 20/20 vision, but your visual acuity can certainly be better than average. Below the 20/20 line lies 20/15, 20/10 and even 20/5. Visual acuity of 20/5 means that what normal vision can discern five feet away, you are able to see clearly at 20 feet.
The Tumbling E
In some instances, patients cannot use the standard Snellan chart. This may be for any number of reasons, such as age or ability. A modified eye chart called the Tumbling E chart is ideal for children who cannot read or verbalize letters yet as well as adults who are illiterate or have a cognitive or speech limitation.
This particular chart uses the same scale as the standard Snellan chart except that all of the letters are capital E with rotations of 90 degrees. The patient points his or her hand, fingers extended, in the same direction as the E on the chart to indicate left, right, up, or down. The ability to read or speak are not needed for accurate acuity testing.