What do the numbers in “20/20 sight” mean?

Popular culture almost always refers to perfect vision as “20/20”, but most people could not tell you what the first or second number means. Optometrists and most places that test for visual acuity opt for the Snellen Chart. We will be going over what the first number means, what the second number means, and what different combinations entail for your sight in practical application.

The first number is based off what the test subject will see at 20 feet away. The test subject always will stand 20 feet away to take their test. This is to normalize results to allow for comparisons. In fact, this means that the first number will ALWAYS be 20(for 20 feet). Normal vision is considered 20/20. This is because when the test subject stands 20 feet away, they can see the same number of letters on the Snellen Chart that the average person can see at 20 feet away. The first number is always used as a reference point for rating one’s visual acuity. It would be impossible to have 10/20 sight if taken in the standard fashion.

The first is static, and the second is variable. The second number represents the distance from the chart that the average person would need to be at to see the same number of letters as the average person. However, if someone had 20/10 sight, this would mean that if they are standing 20 feet away, they can see what the average person can see at 10 feet away. Which means that they have above average sight. Most average people would need to be 10 feet away to read the same number of letters the test subject can read while standing the typical 20 away. The more your number is below 20 feet the better your vision. If you had 20/5, you can be standing 20 feet away, where most people would have to be practically right next to it to see the same as you. If you have 20/100 vision, the average person could stand a third of a football field away and see the same as you at 20.

If someone has astigmatism, it can be hard to include that number in the rating. In the same way, if someone has unique sight problems, it is not always something easy to diagnose. This can include night-blindness. It is very hard to replicate some of the situations in which these start to affect you, even though night blindness is a huge impairment for some people. This number is undoubtedly not a catch all, but it’s a great way to show the kind of distance we can see. In the same way, it is not always the easiest to show how nearsighted you are with this number either. But regardless of any shortcomings, it’s a good way to generalize prescription data, and included in the positive and negative ratings we give both eyes, can give you an idea of someone’s sight abilities.

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