How Do the 20/20 and 6/6 Vision Scales Actually Work?

Ever wonder how the 20/20 and 6/6 vision scales work? Well, wonder no more.

With approximately half of adults requiring some form of corrective eyewear to compensate for visual impairment of various forms, chances are you have had your eyesight graded on the 20/20 or 6/6 scales before. If you haven’t, you have probably heard other people saying they have “20/20 or 6/6 vision” or even the phrase “hindsight is 20/20.” The vision scale is so prevalent that in the U.S. there’s even a TV news show named after it (somewhat ironically when you learn what 20/20 actually implies in a bit) that has aired for almost half a century now.

So what exactly are eye doctors measuring here, what’s the difference between the 6/6 scale and the 20/20, and who came up with the whole thing in the first place?

As to the latter question, enter 19th century Dutch doctor Herman Snellen, who no doubt has long since been reduced to dust, but whose contributions to humanity nonetheless have lived on as more than worm and microbe poop.

Before Snellen, others had come up with a variety of ways to attempt to measure a person’s vision, such as one Benito Daca de Valdes who in the early 17th century was measuring people’s vision based on their ability to see small objects at various distances (things like mustard seeds and even text). Others improved upon this, using text of various size and distances and the like, but there was no standard method that could be easily and accurately reproduced across the globe until 1862 when Snellen published his Test-types for the Determination of the Acuteness of Vision, which included such a standardized method- the famous chart stereotypically topped with a big letter E, consisting of 11 rows of capital letters that get progressively smaller in very specific ways as you go down on the chart.

The major advancement for this chart and method over previous ones included, among other things, Snellen’s custom character set designed to be optimal for measuring visual acuity and calibrating said characters (he called them optotypes) based on external arc, so they could be easily reproduced by anyone across the world.

Snellen’s chart was an instant best-seller, including within a year being picked up by the British Army who wanted a standardized way to measure all soldiers’ visual acuity with reference to one another.

Incidentally, it was also Snellen who came up with the Tumbling E chart that is still also pretty popular even to this day; this one was developed to get around, among other issues with the original chart, the fact that some people, such as miniaturized humans, often don’t know the Latin alphabet. (And further note, many other similar tests have been developed since to try to get around some of the deficiencies of the Snellen chart, but that’s not really important for the discussion at hand, so moving swiftly on.)

This all brings us around to the whole 20/20 and 6/6 thing and what exactly that means. In an overly simplified nutshell, this just means that standing 20 feet or 6 meters (19.69 ft)  away from something, you can see what a so-called “normal” sighted person can see standing 20 feet or 6 meters away from the same thing.  Or more accurately you have the same visual acuity- ability to see the details of something you’re looking at. Or even more accurately than that, measuring your ability to distinguish between two contours that are separated by 1 arc minute (approximately 1.75 mm) at 20 feet or 6 meters.

And, note here, going back to the irony of naming a news show 20/20, first, it turns out what’s actually average is not exactly what Snelling came up with here (he explicitly was going for, to quote him, “easily recognized by normal eyes”, with emphases on “easily”) and thus around 6/5 (20/15) to 6/4 (20/12) would more accurately be the real “normal”, at least until we get particularly close to being worm food.

As you might have guessed from all of this, 20/20 or 6/6 does NOT mean you have “perfect” vision or see things perfectly clearly, as many people say. It simply means you perform in the ballpark of what Snellen considered normal visual acuity, but actually are kind of below average… Which I guess is sort of fitting when talking accuracy and news.

Further it should be explicitly noted that you don’t necessarily have to literally be standing 20 feet or 6 meters away from the chart. The tests by their very nature can be scaled, or sometimes mirrors are used and all that jazz. On top of that, this test, while somewhat practical for judging your vision for certain activities, is decidedly lacking in many ways at actually giving a clear picture of your overall vision and any issues you may or may not have. This is of course why eye doctors have a number of other tests they do.

Alright, so that’s 20/20 or 6/6. What does it mean when you move up and down the chart and the numbers change?

As an example with the Snelling chart, the doctor will ask you to read out the smallest line of letters that you can see from the set distance away. Most people can read the fourth line up from the bottom without any trouble. So if you can do this, again, your vision is considered at least 20/20 or 6/6. Congratulations, you’re at least below average as your parents always knew deep down, Jimmy.

On the extreme bad-vision end, if you can only see the big E clearly (or whatever letter) up top and none of the other lines of text, you are considered to have 20/200 vision (6/60). That means you see at 20 feet (6 meters) what the average person can see at 200 feet (60 meters) away. So to reiterate, 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 many places, such as the U.S.

Alternatively, if you can read the tiny bottom line of text on the chart at 20 feet or 6 meters away, you have 20/5 visual acuity (6/2), which means you can see at 20 feet that which the hypothetical “normal”, but actually a little below average, people can only see at 5 feet away.

Most humans actually don’t have the ability to have much better than 20/10 vision, with 20/5 vision (usually) reserved for animals like birds of prey. But, as noted, plenty of humans dip into the 20/15 or sometimes the 20/10 (6/3) range. And, in fact, for whatever it’s worth, according to Dr. August Colenbrander of the California Pacific Medical Center, the average visual acuity of humans in general doesn’t get worse than 20/20 until we’re collectively in our 60s or so.

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Bonus Facts:

Speaking of vision, a common trope the world over is that humans have only 5 senses… Ya, that’s not a thing in reality. That’s like the “Earth, Wind, Fire, Water” way of looking at senses. Much more on this in an upcoming article. But for now, just know that’s a bunch of crap.

Moving on from there, a common trope is that sitting close to the TV or holding a screen too close to your face will damage your vision… Ya, that’s not a thing either. So why do parents the world over still say this to their kids?  In truth probably mostly so kids will not block the boob tube with their gigantic heads, but also funnily enough there was actually a very brief period of time where sitting close to the TV could damage not just your eyes, but you, assuming you owned a General Electric TV in the 1960s.  Specifically, in 1967, General Electric informed the public that many of their color televisions were emitting excessive x-rays due to a “factory error”.  GE fixed this problem by putting a leaded glass shield around the tubes.

Health officials at the time estimated that the amount of ionizing radiation being given off by these defective TVs was about 10 to 100,000 times higher than the rate considered acceptable.  They recommended, if you owned one of these TVs, not to sit too close.  As long as you were a few feet away and didn’t watch TV for more than an hour at a time or so at this close range, you were probably fine- the 1960s everybody.

General Electric of course recalled all these TV’s and fixed the problem, so the issue went away.

Yet another “shit parents say” is the idea that reading in dim lighting will damage your eyesight. The truth is, it would appear the only “damage” reading in a dimly lit setting will do, in comparison to reading in an ample lighted setting, is to cause extra eyestrain, which will go away simply by resting your eyes.  This shouldn’t be too much of a surprise given the fact that for centuries people have been reading by candlelight without rampant reports of rapidly reduced eyesight.

This particular one even made it on the list of “Seven Medical Myths That Doctors are Most Likely to Believe”- a list put together by the British Medical Journal, which is in turn owned by the British Medical Association (for the other six medical myths that even doctors sometimes perpetuate, these very briefly include needing to drink 8 glasses of water per day, that you only use 10% of your brain, that hair and fingernails continue to grow after death, that shaving affects hair thickness and growth rates, that eating turkey makes you drowsy, and that cellphones used in a normal way will cause problems with hospital equipment- that one was popularized by a 1993 study that has since been thoroughly debunked).

Going back to eyesight, in addition to doctors, 56.3% of teachers surveyed by BioMed Central say that in order to maintain good eye health, people should avoid reading in dim light, despite the fact that to date no scientific study has been able to conclusively show that reading in dim light hurts your eyesight (long term) more than reading in an adequately lit area.

Now there is a slight caveat here- it should be noted that people who read a lot or otherwise focus on things close up for long periods of time, such as people who work on computers all day or do a lot of sewing or the like, do have a higher tendency to develop myopia (nearsightedness), but dim lighting doesn’t appear to make this tendency measurably worse, simply that excessive reading seems to contribute to eventually developing nearsightedness.

Why exactly this is the case isn’t yet fully understood, but the correlation is strong enough between groups of people who do a lot of “close-eye work” and their propensity to develop myopia at a drastically higher rate than the average, that most optometrists are prepared to say that “close-eye work” is for some a major contributing factor to developing myopia.  The leading theory, which seems plausible enough, is that the near constant straining of muscles focusing the eye, stretching the eyeball a bit, over the years gradually causes a permanent lengthening of the eyeball, thus the person developing myopia as they age.

Now, reading in dim light does seem to increase eyestrain, so some hypothesize that this exacerbates the problem, but the consensus among optometrists and the data to date is that if this is what is happening, the difference isn’t going to be so great that it produces a noticeable acceleration of the development of myopia over reading in a well-lit area.

The reason reading in low light is thought to increase eyestrain is because your eyes have to work a lot harder to focus on the words. Your iris is simultaneously trying to open your pupil as wide as possible to let in more light, while your eye is also trying to focus that small amount of light hitting the words onto your retina just right so that you can distinguish between the words and the page itself. This is accomplished by your muscles lengthening your eye even more than normal when reading to bring everything into focus.

Whether reading in low-light or ample light for lengthy time frames, the resulting eyestrain is not serious and one simply needs to rest the eyes on occasion. You can do so by periodically taking a break from focusing on something close up, and instead looking at something far away. Specifically as a general rule, optometrists tend to recommend taking a break from focusing your eyes on close up things for a minute or two every 15-30 minutes.  Also, closing your eyes for a minute helps because, while reading, you typically blink about 1/4 the amount you would normally do, so your eyes can get a bit dry.  Trying to train yourself to blink regularly while concentrating isn’t usually feasible, so the eye-closing method tends to work better for most people.

Conclusion-  Don’t listen to anything your parents say. They are full of crap. 😉 Q.E.D.

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