We all want great vision. Why? So that we can have a competitive advantage. In prehistoric times it gave us the edge to see the wild animals earlier. As society became more organized a method of comparing vision was needed. The credit for the most commonly used method goes to Snellen. Hence the chart which displays the letters or numbers bears his name. It compares a person's vision to the that of the average for the population.
20/20 means that the person whose vision is being tested is normal. If the vision is 20/40 it means that a normal person can see the object at 40 feet away whereas the patient being tested has to go much closer than this to 20 feet before he can see as well. That implies that the test subject has poorer vision. If it is 20/200 than the vision is even worse. Conversely 20/15 means that the test subject can see an object from even further than a normal person. He can see at 20 feet what a normal would see at only 15 feet. 20/10 vision is even better. Lots of professional athletes have vision that is better than 20/15!
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