What Does 20/20 Vision Mean?
Everyone wants to have the best vision possible. Eye doctors also want to provide the best vision possible. Many people think of 20/20 as perfect vision. But many people have no idea what that actually means. 20/20 isn't just arbitrary numbers; it actually has a meaning.
Every eye doctor has some variation of the eye chart. An eye chart has lines of letters that get progressively smaller and more difficult to read. Similar to the picture above.
This is called a Snellen eye chart. Each line on the eye chart corresponds to a certain vision. The big E at the top of the eye chart equals 20/200 vision. The next line down is 20/100 and this continues until you eventually reach the 20/20 line.
To check your vision, this eye chart would be posted 20 feet away (note: the number 20). As you sit or stand 20 feet away from the eye chart, you read the lines on the eye chart until the lines become too small to read. The last line you read becomes your vision; If you were able to read the 20/20 line, you have 20/20 vision!
Fun note: not every room is 20 feet long. When checking vision in smaller rooms, you will either find a room which uses mirrors to reflect the eye chart out to 20 feet or you will have an electronic eye chart calibrated for the room length.
Best corrected vision
20/20 is the average best vision of the population. This is measured using glasses or contact lenses as needed to give the best vision. If you have 20/20 vision, what you can read at 20 feet is the same as what an average person can read at 20 feet (those are the two numbers in the equation); you are the average. If you have 20/40 vision, what you read at 20 feet is what the average person can read sitting 40 feet out. And conversely, if you have 20/15 vision, you have awesome vision since what you can see 20 feet out is what an average person would need to be 15 feet away to see. Checking vision and reading 20/20 on the eye chart is simply how you compare to the average person with your best corrected vision.
Checking vision without glasses
Vision can also be measured without you wearing any glasses or contact lenses. This type of vision is called uncorrected vision. Of note, there are many people who need glasses or contact lenses. 20/20 is NOT the average for uncorrected vision. You don't have to feel bad if you can't naturally see that sharp. You are like many many other people out there who need glasses or contact lenses to see things.
The metric way
If you noticed, the vision measurements are based around feet. You stand 20 feet away from the eye chart. Countries on the metric system actually do things slightly differently. If you convert 20 feet to meters, you get 6.096 meters. So instead of 20/20 being standard, 6/6 becomes the normal average vision!
20/20 doesn't mean everything
Measuring 20/20 on an eye chart only describes one aspect of vision (how sharp you can read letters, called the visual acuity). Visual acuity just happens to be the easiest thing to measure. There is more to great vision than just what you can read on an eye chart. We want more than just sharp vision, we want good quality to our vision. What can effect quality of vision?
- It's very possible to still read the 20/20 line even if you have distortions in your vision. These distortions can come from astigmatism (see also What is Astigmatism?) or other eye disease. You can still read the letters, the letters just don't look very good. The letters may not have distinct margins. The letters may have shadowing to them. These distortions are harder to quantify but reduce the quality of vision.
- It's important to have good contrast when we see things. Having good contrast allows for objects and colors to really pop. If something decreases the contrast in your vision (like a cataract, see also What Are Cataracts?), everything will take on a muted appearance. While you may still be 20/20, you just won't like your vision very much.
- Measuring your vision on an eye chart is like taking a snapshot in time. But life is more like a moving picture. Conditions such as dry eye (see also What is Dry Eye?) can cause the vision to fluctuate between sharp and blurry. This can be exacerbated while doing intense focus activities such as reading or working on computers. While you may measure up to 20/20, you don't experience that clarity throughout the whole day.
- Looking at the end of the room at an eye chart measures your distance vision. It is very possible to be able to see 20/20 in the distance but still be unable to see the computer or read. You are still having vision difficulty despite being "20/20". This happens to everyone at some point in life through a process called presbyopia. The natural lens inside the eye gradually becomes weaker and loses its ability to see up close. This can also occur after cataract surgery with lenses optimized to see in the distance. During these times of life, it is common to not only measure the distance vision but also measure reading vision separately with a reading card.
20/20 is also just the population average. There are going to be some people with healthy eyes that just can't reach 20/20 vision. They may be 20/25 or 20/30 and be perfectly fine since it's the vision that they are used to and which works for them. They just didn't develop a higher visual acuity.
So in the end, 20/20 ends up being just a measurement number. It's useful to track vision over time but isn't the end all be all for vision.
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