How the 20/20 Vision Scale Works

eye2Today I found out how the 20/20 vision scale works.

With more than 150 million people in the United States (nearly half of the population) requiring some form of corrective eyewear to compensate for visual impairment, chances are you have had your eyesight graded on the 20/20 scale before. If you haven’t, you have probably heard other people saying they have “20/20 vision” or even the phrase “hindsight is 20/20.” The vision scale is so prevalent in American culture that there’s even a TV news show named after it.

So imagine my surprise when I was told during my first Australian eye exam that I had 6/6 vision with my corrective lenses. Turns out, the 20/20 scale isn’t universal. So what exactly are eye doctors measuring, and how can the scale be different in different parts of the world?

After examining a large number of people, American ophthalmologists decided on the 20/20 scale, saying that “20/20” is the normal visual acuity of the average person. What that means is that standing 20 feet away from something, you can see what the average person can see standing 20 feet away from the same thing.

Take the Snellen eye chart, which is what your eye doctor will usually use to judge your eyesight. The Snellen chart is the one that’s topped with the big E and consists of 11 rows of capital letters that get progressively smaller toward the bottom of the chart. You will be placed 20 feet away from the chart (most doctors’ offices are too small for this, so mirrors will often be used to simulate 20 feet). The doctor will ask you to read out the smallest line of letters that you can see from 20 feet away. Most people can read the fourth line up from the bottom without any trouble, so if you can do this, your vision is considered 20/20.

Now, obviously most places in the world don’t use the Imperial system to measure distance, they use the Metric system, which is where the 6/6 scale comes in. In this case, doctors are not measuring how well you can see something from 20 feet away, they are measuring how well you can see at 6 meters away (that is, 19.69 feet approximately). It’s the same principle, just a slightly different measurement.

Of course, many people will have worse or even better than 20/20 vision. These people will tip the scales a bit. Back to the Snellen chart, if you can only see the big E up top and none of the other lines of text, you are considered to have 20/200 vision. That means you see at 20 feet what the average person can see at 200 feet away. So, if you take someone with 20/20 vision and put them 200 feet away from the chart, they would still be able to see the big E clearly. 20/200 visual acuity and worse is considered legally blind in the United States. That means that if you have 20/200 vision even with the best correction in your better eye, you are considered legally blind.

Alternatively, if you can read the tiny bottom line of text on the chart at 20 feet away, you have 20/5 visual acuity, which means you can see at 20 feet that which most people can only see at 5 feet away. Again, take that person with 20/20 vision and put them 5 feet away instead of 20, and they would finally be able to see that last line of text. Most humans actually don’t have the ability to have much better than 20/10 vision, with 20/5 vision reserved for animals like birds of prey. These numbers would obviously be adjusted for the 6/6 scale.

That said, your 20/20 or 6/6 visual acuity is not a measure of your prescription as it does not take into account the nature of the problem, only the result of it. That’s why you can’t just pop in to your ophthalmologist, read the Snellen chart, and head out—they have to measure things like peripheral vision, colour perception, depth perception, and eye fluid pressure, among other things.

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Bonus Facts:

  • The Snellen eye chart was created by Herman Snellen, a Dutch eye doctor, in the 1860s. There have been other charts developed, however, which also might be used during an eye exam. An example is  the Tumbling E chart which features capital letter E’s facing in different directions. This chart comes in handy when young children who don’t know the alphabet are being tested, or for people who don’t know the English alphabet. Rather than say a letter, they can pick the smallest line of E’s that they can see, and say or point which way the “arms” of the E in that line are facing. Numerous studies have shown that this chart and the Snellen chart come up with nearly the same results.
  • Around 1 million people aged 40 and older in the United States are considered legally blind.
  • To obtain a driver’s license in the US, you need to have at least 20/40 vision or better. If you can read the fifth line of text on the Snellen chart from 20 feet away, you are considered to have 20/40 vision.
  • Around 2.5 million eye injuries occur every year in the United States, resulting in 5,000 people permanently losing at least part of their vision. Men aged 18-45 are the most likely to have an eye injury occur (over 70% of all eye injuries occur in males).
  • Cataracts, the clouding of the lens inside the eye which leads to decreased vision, affects 22 million Americans aged 40 and older. Around half of Americans have suffered from cataracts by age 80. The medical costs relating to cataract treatment nationwide is estimated at $6.8 billion every year.
  • For colour blindness, red-green defects are the most common, and those of Northern European descent are the most likely to suffer from it. Red-green colour blindness is much less prevalent in other studied populations. Around 8 percent of men and .5 percent of women in Northern European populations have this colour blindness.
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16 comments

  • 2.5 Eye Injuries per year in the US? Not a lot then… 😉

  • Not much I didn’t know here (I’ve had very poor vision – myopic – for most of my life), but I do have a few questions: 1) The 20/20 or 6/6 aren’t used for prescriptions – those are written as decimals, so what is the difference between the two? 2) Why still use the fractional system when (see Q 1)? 3) What law states that 20/200 is “legally blind”?

    Thanks! 🙂

    • Daven Hiskey

      @Maia: I’m not expert on this one, but I think the prescriptions actually specify how to make the lens itself to correct for whatever vision problem you have. The scale really just is a broad classification of the quality of your vision without and with glasses.

  • Try this next time the eye doc asks you to read the smallest line on the chart – just spell out m-a-d-e-i-n-u-s-a.
    My doc at the time had a good sense of humor so he walked up to the chart to see if that text was there – of course he was just humoring me and we had a good laugh.

  • The big “E” is actually 20/400, not 20/200. I’m an optometrist, and I use the chart everyday. You’re right in that the prescription specifies how much power is put into each lens. We measure power in diopters. A plus sign signifies a correction for far-sightedness, and a minus sign is a correction for near-sightedness. 20/200 as legally blind means with best corrected vision, that is with glasses or contact lenses the best a person can see is 20/200 or worse. Many people have 20/200 vision or worse without glasses, but the are correctable to 20/20 with glasses or contacts, so they are not legally blind.

    • what is my vision if i can read the 20/10 line from 30 feet away and the snellen chart is for 20 feet?

      • Well, using the 20/whatever number scale, the first number is always what you can see at 20 feet, and the second at what distance away, in feet, an average person would need to be from the chart to read it as well as you. So, on a 20 foot chart, where normal people can see it at 20 feet, and you can see it at 30, it would be 30/20 vision. But the first number always has to be a 20, because it’s based off your vision, what YOU can see at 20 feet. So we’d need to do some math to correct. I’m only doing this based on the math, not on any optometry charts or equations or know how, because I know noting about that. But simply changing the fraction is easy enough. So we need to go from 30/20 to 20/x. Multiply 30 by .66 to get 20, and multiply the bottom of the fraction with the same number, 20*.66, to equal 13.33. So your vision seems to be 20/13.33 or 20/13 and 1/3. Just standard math, there could easily be some glitch as to why you can’t just straight up treat the 30/20 as a normal fraction and while keeping it the same exact value, just simplifying it so that the top number is 20. But it looks sound to me!

      • I had to chime in on this as the below reply is wrong… I too can read the 20/10 line from 30 feet… and the letters are clear not blurred or jagged at all, suggesting that I may be able to read it from even further back.

        It’s great to hear about somebody else that possesses this amazing gift, I am from England and as I understand this level of visual acuity is very rare in Europeans. (Oddly my dad is considerably short-sighted and my mum has “normal” 20/20 vision so how the hell I got this I don’t know !

        Perhaps it’s a combination of genes that produce this trait, not just one mutation combined with an otherwise healthy (20/20) eye, as in my childhood I did all the things that are said to be bad for your eyes, ie: looking at the TV (CRT) inches away for hours playing Amiga games, reading up close in the dark at night when my parents would turn out the lights, and… staring directly at the sun around midday. I used to time how long I could do it for on my casio remote control watch (real bad idea in restrospect)… but none of this seemed to make any difference.

        If you can read the 20/10 line from 30 feet that means you have 20/7.5 vision, I am no optometrist but if you can read it clearly from that distance – and get every letter right – then I would speculate that you probably have even better vision than that.

        There are Samoans documented to have 20/5 vision, this isn’t just one or two people, this is the average for that populace.

        Congratulations dude ! It’s truly awesome isn’t it and real shame that eye-doctors have never seen such clarity at enormous distances. Ie: they understand that this is possible but they have never actually seen it for themselves.

        Peace mate, enjoy and look after your eyes 😉

        PS isn’t it great to be able to read the small print on a sign that other people can only see the presence of. 😀

        (Or see the flight of a golf ball driven 300+ yards and watch it drop onto the fairway/green. While the other players walk half way up the fairway to get sight of it).

  • Well, apparently Australia still uses the same measurement as the states, except being metric. It doesn’t stop there. In Germany we use diopters. A negative value indicates near-sightedness, a positive one farsightedness. For example you need -1 diopters glasses if you are near sighted and can see things distinctly up to a distance of 1 m, 2 for half a meter and so on (basically an inverse meter). If you are farsighted you need glasses with a refractivity of 2.5 diopters to read a text in a distance of 40 cm (1 / 2.5 m). There are two values, one the left and one for the right eye.

    There is no “English” alphabet BTW. English uses the unmodified Latin alphabet.

  • Can you please make the text large? I have really bad eye sight and cannot read it.

    • Sorry guys, I meant *Larger. Once again I was unable to read my keyboard because my vision sucks, which led me to your website only to find out the text is to small to read because my vision sucks.

  • Surely the Australian 6/6 system is just the metric equivalent of the old fashioned US system.

    20 feet is more or less the same as 6 metres.

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