What does 20/20 mean?

Chan Optometry |

20/20 is essentially a comparison between you and someone with “normal vision”. The first 20 is you, standing 20 feet away looking at a very small object. That doesn’t change, always stays the same. The second 20 is the person with “normal vision”. You’ll notice that I put normal vision in quotes – essentially, having 20/20 vision does not necessarily mean your eyes are normal – there are a lot of disorders/diseases of the eyes that would allow you to still have 20/20 vision but they are definitely not normal. But I digress. The second person is standing right beside you, looking at the same object at 20 feet away. If you can see that object clearly, you have 20/20 vision.

Confusing right? Let’s do another example to help clear up the muddy water. Let’s say you have 20/30 vision – which is a little poorer than 20/20 vision. Again, you don’t change – you can see the same small object, standing 20 feet away. Hence the first “20”. Now that normal person, he or she can walk backwards to 30 feet away and STILL SEE THAT SAME OBJECT CLEARLY. That means that the object must be bigger than in the first example (or else you won’t be able to see it).

An extreme example: 20/400 vision. Again, you can see an object clear at 20 feet away, but that normal person can walk backwards to 400 feet away and STILL SEE THAT SAME OBJECT CLEARLY. You know that big E that we put up at the beginning of the vision test? That’s 20/400 vision.

Now, some people would say, “Oh I can’t see that E without my glasses or contacts. I’m legally blind!”. Keep in mind we test for legal blindness best corrected. That means with your glasses/contacts on, not off. Now, if you still can’t see that E even with your best corrected glasses/contacts, then, well, you should get your eyes checked to see what’s going on.

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