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What is “20/20” vision?

By Rebekah Warner | 7/26/17 7:16 AM
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Commentary by Joel Hunter, MD

This article discusses the philosophical side of vision measurement. It’s something we don’t think about a ton (or maybe it’s just me), but ultimately our measurements of all things are usually pretty arbitrary. Why is a meter the exact length that it is? Why is a second as long as it is? The amount of time in a second is actually interesting enough to be its own article, and if the public outcry is vociferous enough, I am happy to write it. For now though, here’s Rebekah’s article about 20/20, and my editorialization as an ophthalmologist.

We all know that to have 20/20 eyesight is considered optimal. But what does “20/20 vision” even mean? Who decided on this standard of vision perfection? And is it possible for some people to see better than 20/20?

In the days before vision charts, a standard vision test (called something like “ye olde vision test,” I’m assuming) was described by being able to see the separation between two specific stars in the handle of the Big Dipper. That sounds like a made up fact, but is actually true. Mizar and Alcor are the stars. It’s interesting not just for the trivia, but because it underlines that good vision had a much, much more forgiving standard in the pre-industrial age. Being able to tell those stars apart requires 20/60 vision, which is 300% worse than 20/20.

Believe it or not, this level that describes “perfect vision” is not even reachable by a lot of American people. According to one news report, about half of adults in the United States don’t even see as well as 20/20. Does this mean the bar is set too high?

This helps explain why those stars were the standard back then, I suppose. Not a lot of people have naturally good vision. The bar is higher than olden times, but that’s because we can do a lot more to help vision now.

The definition of 20/20 vision

To put it plainly, to see 20/20 means a person can interpret an image from 20 feet away that is meant to be distinguishable from that distance. Hmmm. Maybe that’s not as clear-cut as I had hoped.

Let’s go back to the Snellen chart. The Snellen chart, which you’ve probably seen a time or two, was created in the 1860s by Dr. Herman Snellen of the Netherlands. These are the eye charts most often seen in schools or medical facilities with a big letter “E” at the top, followed by lines of consecutively smaller letters.

By the way, if you’re an ophthalmologist, pretty much every gift you get from someone for the rest of your life will in some way involve a Snellen chart, usually with a motivational saying written out in the letters.

When a person’s vision is tested using the Snellen chart, they stand 20 feet away from the sign and read it from the top down. That letter “E” at the top? That is allegedly recognizable from 200 feet away. If that is the extent of the chart that a person can identify from 20 feet away his/her vision is said to be 20/200. A few lines (nine lines) further down the chart from there is the target “20/20” line. Yes, this seems somewhat ambiguous, huh? With his chart, however, Dr. Snellen was able to create a method for measuring eyesight and explaining visual acuity. If it’s not broken, why fix it? For that reason, Snellen’s system is used to this day.

Interestingly, the Snellen chart and letter sizes aren’t arbitrary. A healthy human retina can distinguish a difference of one minute of arc. A minute of arc is 1/60th of a degree. The retina is the specialized light-sensing tissue that lines the curved inside of the back of the eye. If you were to shrink down to a microscopic size and stand on a retina, a 20/20 letter E would be project across 5 minutes of arc of this globe of an eye you’re standing inside. That’s because the top, middle, and bottom lines of that E would each be separated by one minute of arc spaced between them. That E from top to bottom is three dark bars, separated by two white spaces, each of them one minute of arc (so five total). Some algebra can be used to calculate that a letter this size on the retina would be 0.35 inches tall on a chart 20 feet away.

We should mention that 20/20 is not a universal guideline. It is pretty much just an American criterion. In many countries where the metric system is primarily used the reference point is 6 meters, with perfect vision being 6/6.

Like inches and pounds, the US stands pretty solitary on this one. Most international vision research and testing is done with 6/6 as the ideal vision. None of the math on letter sizes changes though. A 6/6 letter size on an international eye chart would still be 0.35 inches tall on a chart 20 feet away. They just use eye charts 6 meters away because they’re not uncivilized Americans.

Does anyone see better than 20/20?

Yes, seeing better than 20/20 is possible. After the 20/20 line on the Snellen chart, you will notice a couple additional lines. What are those for, you ask? Well, they go on to measure if someone has better than 20/20 vision. For example, a person who reads from 20 feet what others need to be only 15 feet away to see has 20/15 vision, aka eagle eyes. No, in actuality, the differences between eagle vision and human vision are a bit more complicated. Birds of prey have more like 20/2 vision.

Birds of prey have incomprehensibly good vision. It’d be fun to say that we are getting there with human vision, but it would also be untrue. It’s an eye that is built fundamentally different than a human eye. It’s like comparing a DSLR camera with a 200mm lens to a Funsaver disposable camera like the one you might find on the table at a wedding reception.

You lost me back at the last “Snellen chart”

Ok, let’s leave that discussion for the birds. No, it’s not likely you’ll ever achieve the visual acuity of a predatory fowl. On the other hand, a recent clinical study reports around 94% of the participants, all of which had received LASIK vision correction, seeing at or better than 20/20 within 6 months of the procedure. Not too shabby.

This study significantly undershoots what is possible for accuracy and precision with the right laser and the right pre-operative diagnostics. At Hunter Vision, good candidates getting LASIK for distance vision have a 20/20 outcome 99% of the time. The average distance vision for our post-op patients is 20/18.

The likelihood of attaining so-called perfect (or better) vision isn’t the only thing to think over when considering LASIK, however. As with any medical procedure, there is a possibility of side effects. A couple of the more common ones with LASIK surgery include dry eye or visual disturbances, such as halos, starbursts or glare.

Dr. Dan Durrie, my idol and mentor, must have told me 1000 times during my fellowship, “LASIK is 95% knowing what to do, and 5% doing it.” He meant that you have to make absolutely sure that you’ve covered every base with your diagnostic testing beforehand to avoid these complications. Honesty on the front end of things is worth its weight in gold when it comes to sparing people from side effects.

Even so, if you are in that 50% of U.S. adults with sub-par eyesight and would like to discuss your options for vision correction, we are happy to get that conversation started. We have experienced LASIK surgeons here to answer your questions and address any concerns you may have.

This includes any questions about the definition of 20/20. I sent out a copy of the paragraph explaining that to our staff to ask if it was comprehensible and no one wrote back. That seemed like a bad sign.

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