20/20 Vision

I have 20/20 Vision!

 During an eye examination, eye doctors use eye charts to gauge how well you see in the distance. The optometrist will ask you to find the smallest line of letters that you can make out, and ask you to read it. If you can read the bottom line of letters, that means you have a good visual acuity. The doctor might say, you have a 20/20 vision, but what does 20/20 vision means? 

20/20 Defined

What 20/20 Vision In An Eye Test Means The name “Snellen” came from Dr. Herman Snellen’s surname, a Dutch ophthalmologist who invented the Snellen chart. In the United States, the standard placement of the eye chart is on a wall that's 20 feet away from your eyes. Since many eye doctors' offices don't have rooms that are 20 feet long, in a smaller room the eye chart may hang behind the patient chair, using mirrors to make it appear in front of you at a simulated distance of 20 feet.

 How a Snellen eye chart and a "tumbling E" chart might look at your eye doctor's office. The tumbling E chart tests the visual acuity of young children and others who can't read letters aloud.

20/20 vision (or really, 20/20 visual acuity) is considered "normal" vision, meaning you can read at 20 feet a letter that most human beings should be able to read at 20 feet. Eye charts can be configured in various ways, but generally, if during an eye test you can read the big E at the top but none of the letters lower than that, your vision is considered 20/200. That means you can read at 20 feet a letter that people with "normal" vision can read at 200 feet. So at 20/200your visual acuity is very poor. In the United States, you are considered "legally blind" if your best-corrected visual acuity (meaning, your best distance vision with eyeglasses or contact lenses) is 20/200 or worse.

To get a driver's license in most of the United States, your best-corrected visual acuity must be at least 20/40. 

Usually the 20/20 line of letters is fourth from the bottom, with 20/15, 20/10 and 20/5 below that. Not many people have 20/10 or better visual acuity, but many animals do, especially birds of prey, which have been estimated to have an acuity of 20/5 or even better. (All About Vision)