What is 20/20 Vision? How to Convert Diopters to a 20/20 Measure
We all hear the term 20/20 vision thrown about to mean anyone who has healthy eyesight. But what does it mean? And how does it convert to the lens prescription your optometrist gives you, measured in diopters? By the end of this article you’ll be an expert on the definition and understanding of 20/20 eyesight.
What is 20/20 Vision?
The average human being with healthy eyesight has 20/20 focus. This is a benchmark measure of what you should be able to see at a distance of 20 feet.
This measure only really becomes important when people stray from the norm. For instance, if you have 20/100 vision, it means that other people can see clearly from 100 feet away, what you can only see at 20 feet. It’s a big disadvantage.
Can You Have Better Than 20/20 Eyesight?
Yes, it is possible – having 20/20 focus is merely the average visual acuity for a healthy human being. But there is nothing stopping people from being above average and gaining better than 20/20 vision.
Some fighter pilots use eye exercises to attain 20/10 vision, while the world record belongs to a man naturally achieved 20/8 focus by training his eyes for better vision. Incidentally, eagles have natural vision of 20/2 – hence the expression eagle eye, which means an exceptional eye for detail.
What are Diopters?
If you are nearsighted (anything worse than about 20/50 vision), your optometrist will write you a prescription to improve your long distance vision. This describes, in a measure called diopters, the refractive power of your new glasses or contacts.
You will get a separate diopter value for each eye – often one eye is slightly weaker than the other. If the measurement has a minus (-) before it, you are nearsighted and have trouble seeing things in the distance. If there is a plus (+) before it, you are farsighted and have trouble seeing things up close.
How to Convert Diopters to 20/20 Vision
While the 20/20 vision system measures how well you can see compared to a healthy benchmark, diopters measure the focusing power of your glasses.
There is a relationship between the two, shown in the table below. This gives you a very rough conversion between diopters and the 20/20 measurement – but bear in mind there is a lot of variation between individuals.
|
Diopter Measure
|
20/20 Measure
|
|
-1.00
|
20/40
|
|
-2.00
|
20/80*
|
|
-3.00
|
20/150*
|
|
-4.00
|
20/300*
|
|
-5.00
|
20/400*
|
|
-6.00
|
20/500*
|
*Higher diopter values are harder to calculate because it depends on the person – some sources cite a whole range between -4.00 and -6.00 to reflect 20/400 vision. So these figures are an approximation. Note how the 20/20 measure doesn’t increase consistently, and may actually increase exponentially beyond -6.00 (severe myopia).
