We all have to aspire to something. Perfection? It’s very hard – probably impossible – to attain. Well, what about “perfect vision”? Isn’t that something attainable?
Um. I… ah. Not exactly.
But, wait! 20/20! ISN’T THAT PERFECT VISION??
I… No. Not really.
So, speaking professionally, what exactly is 20/20? Is it “perfect vision”? Is it “normal vision”? What does it mean??
Well… It’s complicated. And, as with so many other situations in this field, we ask our patients not to get hung up on numbers. That being said, try this on for size:
20/20 is what is used to measure visual acuity or the clarity or sharpness of vision. If a person has 20/20 vision, they would be able to see clearly an object 20 feet away that should be seen from 20 feet away. Still a little perplexed? Understandable.
The bottom line is this: The people in charge of such things needed a number to describe “normal”. A number to use as a baseline. 20/20 was it.
Now, does that mean that having 20/20 vision means that your visual acuity is “perfect”? Nope. It’s only an indication of a good ability to see details. Other things like peripheral – or side – vision, focusing ability, depth perception and color vision all contribute to your overall vision.
For instance, if you are hyperopic, or farsighted, you may be able to see pretty well in the distance for driving or TV, but you can’t focus very well up close for reading or cell phone use.
Conversely, if your uncorrected visual acuity is 20/80, then you have to be 4 times closer to an object than a person with 20/20 vision.
You following all this?
And, just to throw a monkey wrench into the works, I’m going to tell you about being “legally blind”.
Every-so-often, we will get a patient who will respond to a given question by saying,
“I’m legally blind without my glasses!”
Number one, a good many of us would be if not for number two, which is: That’s not how it works.
The US definition of legal blindness is this: “A best-corrected visual acuity of 20/200 or less in the better seeing eye.” Best-corrected meaning the visual acuity when wearing glasses or contact lenses.
A person who is correctible to 20/20 with glasses or contacts will not be considered “legally blind”.
And, to clarify, 20/200 means that a person with that acuity must be 20 feet away from an object that a person with “normal vision” can see clearly from 200 feet away.
There is another indicator that I won’t dive too deeply into. If a person’s field of vision is restricted to 20 degrees diameter or less – often called tunnel vision – they could also be considered legally blind. For comparison, the average person has about 160 degrees of peripheral vision without rotating their head.
The Centers for Disease Control and Prevention states that more than 3.3 million people over age 40 are considered legally blind or to have low vision. Low vision, in this case, means a best-corrected visual acuity of 20/70 or less in the better eye.
What about total blindness?
To be totally blind is to have no perception of light or form and it’s uncommon. Only about 15% of people with legal blindness are considered to be totally blind.
What causes a person to be blind?
There are many causes. Blindness can be congenital, or from birth. It can be caused by some form of trauma or injury. The majority of people who “go legally blind” suffer from one or more of these sight-robbing conditions:
Age-related macular degeneration,
There is another condition that can lead to legal and, sometimes, total blindness and that is Retinitis Pigmentosa or RP.
RP is genetic. It is passed through bloodlines and it is rare. It’s estimated to affect 1 in 4,000 people worldwide. The flip side is the 11 million people in the United States alone that have some form of macular degeneration. The worldwide number is estimated to reach about 196 million this year.
Sound scary? It is! The problem is that catching these conditions by waiting until something is noticed by the patient is difficult because they are virtually without symptoms until they have advanced significantly.
This is why we stress so strongly the importance of an annual comprehensive eye exam.
We aren’t just trying to provide you glasses or contacts. We are trying to save your sight. Especially if you have a family history of Retinitis Pigmentosa, Macular Degeneration, Glaucoma or diabetes.
Dr Kay and Dr Deweese are always here to keep your eyes safe and healthy.