What Does 20/20 Vision Mean? What is 20/20 Vision?
April 5, 2021
What does 20/20 vision mean? What is 20/20 vision?
This is one of the most common questions eye doctors get. There are several different ways of answering this, but the easiest is to imagine yourself 20 feet away from a street sign. If you can just barely see the letters, and you have good vision, you probably have 20/20 vision. If someone is with you and they have to get 10 feet away from the letters in order to just barely see them, they probably have 20/40 vision. It’s a fraction meaning “you you need to move in to 20 feet to be able to see what an average healthy person can see at 40 feet.” Conversely, if you have 20/15 vision, you can see at 20 feet what an average healthy person can see at 15 feet. Congrats, you’re seeing a bit better than average.
“I’m like legally blind, like 20/2000!”
As eye doctors, we’re not too concerned with your uncorrected vision, but more concerned with your best-corrected vision, and with your refractive error, which we measure in units called “diopters.” For the very interested (geeks/nerds-hey, I’m one also!) out there, a diopter is D=1/f, where f=the focal length in meters. So if your contacts are -4 Diopters, then -4=1/f and f=0.25 meters, which is 25 cm, or about 10 inches. What this means is that without your glasses on, your vision will start to blur at about 10 inches away from your eyes. If you’re -10, then your vision starts to blur at 1/10th of a meter, 10cm, or about 4 inches away from your face. So it’s a huge difference in glasses thickness between -4 and -10, but only 6 inches of additional clear vision.
What’s astigmatism got to do with it?
Astigmatism, also called cylinder, occurs when the shape of your cornea (the clear dome of collagen over the colored part of your eyes) is not perfectly spherical. If I cut a tennis ball in half, then squeezed the sides together, it would create the same shape as astigmatism. Modern laser vision correction has been correcting astigmatism since 1998. I’ve even corrected up to 8 diopters of astigmatism with the laser, which is really unusual, but potentially treatable. Astigmatism also degrades the image quality of whatever it is you’re looking at, but in a slightly different way. With nearsightedness, also called sphere, the vision is just blur. With astigmatism images are distorted so that sometimes one can function, drive etc without glasses if they’re not also nearsighted, but they may have difficulty seeing road signs, may have ghost images (a second, lighter image next to the first) or get headaches and see strain when not wearing their glasses. You need a complete (free!) exam at SharpeVision to assess everything and see if you can be one of the fortunate to have LASIK or another vision correction procedure so you won’t need glasses or contacts.
Why do I have 20/20 vision far away, but can’t see my phone? It’s annoying!
I’m gonna go out on a limb, and say that if you have this problem, you are over 45 years old. There’s a condition that affects us all called “presbyopia” (prez-bee-OH-pee-uh) which is due to the loss of flexibility in the lens of our eyes that affects us at about age 45 and older. After about age 45, without your glasses, you can see either at distance OR near, but not both (and sometimes neither in the case of astigmatism or farsightedness)! If you have the ability to see distance, but not near, you’ll need readers (a.k.a. cheaters) bifocals, multifocal contact lenses, or multifocal intraocular lenses with cataract surgery in order to be able to see near objects. Hopefully within a few years, there may be eye drops that help with presbyopia:
What’s the best vision? What’s the worst vision?
The best vision ever recorded was allegedly 20/8 in recently deceased (December 2020 at age 97) test pilot Chuck Yeager. A very small number of people (about 3-5%) are capable of a best corrected or even uncorrected 20/10-those are super tiny letters. Most (about 65%) healthy young people are capable of 20/15 vision. 20/20 vision is just an average, with many reasons why it’s slightly less than 20/20. On a vision chart, the lowest/worst vision (biggest letters) is usually 20/400, which means that you need letters 20 times as big as the 20/20 letters. Pretty bad. But it unfortunately can get worse in a variety of pathologic conditions. When the best corrected vision is worse than 20/200 in the better eye, we call that legally blind, and is important for being eligible for government aid. When the vision is worse than 20/400, we measure it in “count fingers” at a certain number of feet, and is denoted as “count fingers at 4 feet” for instance. If one cannot see well enough to count fingers, we progress to hand motion, light projection (they can tell what direction the light is coming from), light perception, and no light perception. This is very sad when someone has no light perception at all, because it’s not likely reversible.
Who invented this?
This standard had been around since 1862 when Dutch ophthalmologist Herman Snellen devised his famous “Snellen Eye Chart” that is reportedly the most sold poster of all time. More even that that Einstein poster with the wild hair, and the “Keep Calm and Carry On” posters combined!
Why is it necessary?
Much like the “vital signs” of your body, such as blood pressure, heart rate, and breathing rate, eye doctors need to know your uncorrected visual acuity and your best corrected visual acuity at distance and near. This tells them what you are capable of seeing, and helps with assessing the health of your eyes. There are many other important elements to your eye health, such as eye pressure, that can help determine the risk of glaucoma, and other anatomical measures of your eye health. So while your visual acuity is important, it’s just one piece of your eye health picture.
I’m not from the US. I’m not familiar with the term “20/20 Vision.”
In Europe, vision is measured in meters in a 6/6 meters format, which is pretty close to 20 ft. In Asia, it measured as a fraction. 1.0 vision is 20/20, 1.33 is 20/15, or a bit better than average, 0.5 is 20/40, or a bit worse than average.
I have 20/20 Vision. Does this mean my eyes are perfect?
People often think that 20/20 vision means they have perfect eyes. This can be dangerous, because there are many aspects to eye health. Visual fields, color vision, eye pressure, eye movements, pupillary function, the surface of your eyes, eyelids and skin, and many other components of your eyes make up a healthy visual system. So asking “Is your vision 20/20?” Is more like asking if you have normal blood pressure. It’s simply one of many aspects of your visual system.
20/20 vision meaning is more than just a fraction or feet or something to aspire to, it’s part of not only your eye health, but your entire body’s health.
Why don’t I have 20/20 eye vision?
There are many reasons for having less than 20/20 vision in each eye individually. At SharpeVision, the most common reason for less than 20/20 vision is “refractive error” (some nearsightedness, farsightedness, astigmatism, or a combination of them). However, it’s extremely important to have a complete eye exam to determine the cause of blurry vision. The list of reasons for blurry vision is nearly endless. The most common reasons for blurry vision are:
- Refractive error: nearsightedness, farsightedness and /or astigmatism: Give us a call at 425-451-2020 or schedule online at sharpe-vision.com
- Cataracts: almost everyone will get these at some point in life. Typically about age 70 is when people start having enough clouding of the natural lens in their eyes that they have glare at night, or can’t get 20/20 vision even with glasses. If this is the case, an evaluation with an ophthalmologist eye surgeon is necessary.
- There are hundreds of other causes ranging from annoying (allergies) to seriously vision threatening (retinal detachment or infection) that can be assessed by your eye doctor.
Why is having 20/20 vision important?
Having good vision is obviously important in every single thing we do in our lives, from work and play, to driving and reading. 20/20 vision is an important part of eye health, but I can’t over emphasize the importance of routine screening eye exams. I always remember a patient I had when I was a resident years ago. He was in his 40s and had a perfect 20/15 (better than average) vision in each eye, but he had very advanced glaucoma with dangerously high eye pressures when he first came to see us. His visual field test showed what is called “shotgun barrel” fields. He could only see things straight ahead, and had lost all his peripheral vision. Sadly, in spite of maximal therapy and surgery, he lost all vision over the course of the next several years. I write this to maybe spur someone reading this into action.
“I’m legally blind!”
When someone says, “I think I’m legally blind my eyes are so bad,” remember that legally blind is a legal definition, not a medical one. It means that they qualify for government aid. Having thick glasses is not legally blind. The average refractive error (glasses prescription we treat for laser vision correction is -4 diopters (units of measure) for the nearsighted component, and -1.00 diopters of astigmatism. However, we can treat much higher with either LASIK, PRK, ICL (implantable contact lens), RLE (refractive lens exchange) or cataract surgery.
So the next time someone says, “What does 20/20 vision actually mean?” you’ll be able to tell them!
Dr. Matthew R. Sharpe
