20/20 Vision: What is it?Author: Dr. William Tullo
December 12, 2013
20/20 vision is a term most people have heard. But just what is 20/20 vision?
It’s how well you see compared to a person with so-considered “normal” vision. Here’s what that means when applied to a scenario:
Meet Daniel. Daniel, with normal vision, goes to his eye doctor for his annual visit. He sits in a chair 20 feet from a wall, and his eye doctor puts up a chart of letters for Daniel to read. (The chart is called a “Snellen” chart, named for Dutch Dr. Hermann Snellen.)
Daniel can read with ease from the big “E” at the top of the Snellen chart down the progressively smaller lines to the eighth line, which might or might not be the last line. Daniel has 20/20 vision.
There are a couple ways to think about Daniel’s 20/20 vision. If you simplify 20/20, you get one whole number—a perfect mathematical situation—which we can think of as normal, in other words. At 20/20, Daniel’s vision is normal.
You can also think about this by considering that a person with 20/20 vision can see at 20 feet (the first “20”) what any normal person could see at 20 feet (the second “20”).
Now let’s change it up.
Far from 20/20 vision: Legally blind
Here’s another scenario: Cindy sees half as well as Daniel. Cindy’s vision is 20/40, which simplifies to a half.
When Cindy goes to the eye doctor, she needs to get 10 feet from the Snellen chart—half as close as Daniel had—to see with ease from the big “E” at the top to the eighth line. When she sits in the chair, she can see only half as well as Daniel.
If Daniel can read a sign at 40 feet, Cindy would have to move to 20 feet to see it clearly. In other words, Cindy sees at 20 feet (the first “20”) what Daniel sees at 40 feet (the “40”). That gives us her 20/40 vision.
So what about Johan, who has 20/200 vision?
Johan has a big disadvantage. Johan has to be 20 feet or closer to read a sign Daniel could see from 200 feet away. Johan’s vision is 1/10th as good as Daniel’s.