What is 20/20 Snellen fraction?
When we take a driver’s license, we’ll get a number like “20 /?”.This is our visual acuty.But how can we know how much the question mark is? It’s very simple. I’ve figured it out for everyone, please see the table below:
And what does the “20/20” mean？Visual acuity can be checked by reading a Snellen chart from a distance of 20 feet (1 foot = 0.3 meters). By observing a large number of people, ophthalmologists have mastered what should be seen on the eye chart at “normal” vision at 6 meters. If your vision is 20/20, you can see what you can see with “normal” vision at 20 feet from the eye chart. (In metric conversion, the standard is 6 meters, so it is also called 6/6 vision.) In other words, if your vision is 20/20, it means that your vision is “normal”, that is, at 20 feet, the Most people do what you see.
If your vision acuity is 20/40, it means that a person with normal vision can see something 40 feet away from the eye chart, and you need to stand 20 feet to see it. 20/100 means that what others with normal vision can see at 100 feet away from the eye chart, you still need to stand at 20 feet to see it. In the United States, vision is considered blind if it is below 20/200.
There are also some people with vision above the normals. People with vision of 20/10 can see what is 20 feet away from the eye chart, and normal people can see it only 10 feet away.
Reference:
http://www.topa8.com/index.php?Article-index-id-3368.html
https://wenku.baidu.com/view/d2477caa10661ed9ac51f303.html