In this particular episode of Ask an Eye Doc, you will learn:
- What 20/20 means
- What is visual acuity
- The fallacy of “perfect vision”
- What is “standard vision”
- Why trigonometry is a prerequisite for optometry school
For you readers out there, here’s the answer in written form:
You’ve probably heard that 20/20 vision means you have “perfect vision”, but what do those numbers actually mean?
The term 20/20 is a measurement of visual acuity. Visual acuity measures how blurry your vision is by determining the smallest print size you can read on a visual acuity chart.
The first number is the distance at which your vision was tested. In the United States, this is 20 feet from the visual acuity chart. (In Europe, it’s 6 meters). The second number is the distance a person with standard vision can stand from the chart and still read the same print size that you were able to read.
If your vision is 20/40, a person with standard vision can read from 40 feet what you can read from only 20 feet. This means your vision is twice as blurry as someone with standard vision.
If you have 20/20 vision, you can see the same letters on a vision chart that a person with standard vision can see. Sometimes this is mistakenly called “perfect vision”.
The fallacy of “perfect vision”
Some people have 20/15 vision, meaning they can see letters from 20 feet that a person with standard vision can see from 15 feet away. This is why 20/20 can’t be perfect vision, since 20/15 would be better than perfect, and that just doesn’t make sense. So the correct way to think about 20/20 vision is “standard vision” rather than “perfect vision”.
Herman Snellen’s “standard vision”
This leads to an obvious question: Who created the current definition of “standard vision”? It was a Dutch ophthalmologist named Herman Snellen. In 1862, he developed the Snellen visual acuity chart, which is still widely used today. After using his chart to measure his patient’s vision, he defined standard vision as the ability to recognize one of his optotypes (or print letters) that subtended 5 minutes of arc.
What?! I’ll do my best to explain…
If you know about trigonometry and angles, you’ll know that one minute of arc is the same as 1/60th of a degree. Think of the corners on a square. Each angle is 90 degrees. If you draw a diagonal line through the square you get a triangle that has a 45-degree angle and two sides that are the same length. Divide that angle by 45 to get a 1-degree angle, and then divide that 1-degree angle by 60 to get a 1 minute of arc angle. We’re talking about a really small angle!
As the angle gets smaller and smaller, the height of the triangle also gets smaller and smaller. In Snellen’s definition of standard vision, we know the angle is 5/60th of a degree and the testing distance is 6 meters (because he was European).
With those numbers, we use one of the trigonometric functions called tangent to figure out the height of the triangle, or in other words, how tall the letter should be. According to my calculator, that’s about 8.73 mm tall. So another way to define standard vision is being able to see an 8.73 mm tall letter from 6 meters away.
Why did Snellen choose 5 minutes of arc? Well, he actually chose his optotypes (print letters) that were 1 minute of arc thick. Think about a block letter “E”. The E has three horizontal arms and two spaces in between each arm. Each of the three arms and each of the two spaces is tall enough to subtend 1 minute of arc, making the total height of the letter E tall enough to subtend 5 minutes of arc.
So another way to define standard vision is being able to distinguish stripes that are 1 minute of arc thick, separated by 1 minute of arc of space, like the 5 minute of arc block E.
Wow, I really got into the weeds on this one! Thanks for hanging in there for this answer. And this is why trigonometry is a prerequisite for optometry school…