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October 19, 2016

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Are you seeing 20/20?

October 19, 2016

A lot of patients ask me at the end of their eye exam "So is my vision 20/20, or what?" Or for some near-sighted folks "So am I blind, or what?" 

 

The first thing to know is that 20/20 is a measure of visual acuity (the ability to see fine detail), and it is a very specific measurement under specific conditions. So just because I measure your visual acuity to be 20/20 in my exam room, doesn't necessarily mean you are seeing things clearly in your every-day life. There are usually other variables involved, such as the lighting situation or whether your glasses need to be cleaned. 

 

The other important thing to know is that we Optometrists generally measure what is called "Best Corrected Visual Acuity" or BCVA for short. "Best Corrected" means that we measure your acuity while you are wearing your glasses or contacts

 

So if you are wondering if you are legally blind, the answer is probably not unless you can't see anything clearly even WITH your glasses or contacts on.

 

Anyway, back to 20/20. And here's where it gets interesting...kind of.

 

Way back in the day, this guy named Snellen developed a visual acuity chart (the one you always think of with the big "E" at the top and numbers getting smaller going down). He put this chart a standard length of 20 feet away from his patients (that's 6 meters for those folks across the pond).

 

A person with "normal" vision standing 20 feet away from a Snellen chart should be able to see the letters which subtend 5 minutes of arc upon their retina.

 

Stay with me here. 

 

"Minutes of arc" is a fancy way of saying "how much space this thing takes up on your retina". The letters on the 20/20 line of the Snellen chart, when viewed at 20 feet away, should be the smallest thing a "normal" human eye can physically perceive.

 

Some people have better than 20/20 vision. Someone with 20/15 vision would be able to see at 20 feet what a "normal" person could only see at a closer distance of 15 feet. Many young people have better than 20/20 vision. And usually brag about it.

 

And other people have worse than 20/20 vision. For example, someone with 20/200 vision can see clearly at 20 feet what a "normal" sighted person could see from 200 feet! Remember, these measurements are taken as Best Corrected Visual Acuity. It is rare for someone to see as poorly as 20/200 while wearing glasses or contacts. Usually this poor of vision would represent an injury or disease process. 

 

As an interesting aside, most states require 20/40 Best Corrected Visual Acuity for driving. Wisconsin allows up to 20/100 best corrected. So a 20/100 person driving around in Wisconsin can see at 20 feet what a "normally" sighted person can see from 100 feet away.

 

Drive safe, people. 

 

 

 

Image credits:

By Jeff Dahl - Own work by uploader, Based on the public domain document: [1], CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=4262200

 

http://webvision.med.utah.edu/book/part-viii-gabac-receptors/visual-acuity/

 

 

 

 

 

 

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