20/20 Vision Explained by Tangney Opticians in Tralee.
Eye opticians use eye charts to measure how well you see in the distance.
There are many variations of the Snellen eye chart, but in general they show 11 rows of capital letters.
The other rows contain letters that are progressively smaller. During an eye exam, your eye doctor will ask you to find the smallest line of text letters that you can make out, and ask you to read it. If you can read the bottom row of letters, your visual acuity is very good.
What 20/20 Vision means in Eye Tests.
The standard placement of the eye chart is on a wall that is 20 feet away from your eyes.
Since the eye test doctor offices don’t have rooms that are 20 feet long, in a smaller room the eye chart may hang behind the patient chair, eye tests opticians may use mirrors to make it appear in front of you at a simulated distance of 20 feet.
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. At 20/200, your visual acuity is very poor.
Legally blind is if your best-corrected visual acuity (your best distance vision with eyeglasses or contact lenses) is 20/200 or worse.