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Ask the Doc: What Does It Mean to Be Legally Blind?

June 23, 2016 | By Dr. David Gibson

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You've probably heard of the term "legally blind", but do you know what it actually means? It's kind of confusing, so we called in one of our independent optometrists to shed light on the matter. Here's what he had to say:

glasses-272399.jpg"A lot of patients will come to see me and tell me that they've been diagnosed as legally blind by a previous eye care provider. I can't help but cringe when I hear this, because it is usually completely incorrect and trivializes a serious problem that affects many people profoundly." - David Gibson, O.D.

There's an estimated 7 million legally blind people over the age of 16 in the United States. So what does it mean?

Legal blindness is defined using two criteria, only one of which you need to meet to be accurately classified as legally blind.

1) How well you can see via the acuity chart: most of us are familiar with that big chart of letters (don't try and memorize it!) that the eye doctor asks us to read aloud (typically with instructions like: "Read the lowest line that you can.") That's the acuity chart we're talking about. 

Someone who is legally blind will only see 20/200 out of their best eye when they are wearing their glasses or contacts, even with the strongest, most accurate prescription possible. Of course, this also means the weaker eye will see worse than 20/200 with the best possible prescription.

On the other hand, someone who sees 20/20 while wearing the correct vision wear, but sees 20/200 without it, is not legally blind. It is only when you are fully equipped to see perfectly, but still cannot. 

2) Your visual field: if your visual field - the total area you can see to the side while looking straight ahead - measures at 20 degrees or less, you are legally blind. Think of this as a test of your peripherals. For comparison's sake, the "normal" visual field is 135 degrees. Those who are legally blind because of decreased visual field have severe tunnel vision, but still may be able to see 20/20, unlike criterion #1. 


There you have it - the true definition of legal blindness straight from the mouth of an experienced optometrist. We hope this cleared things up for you (pun intended)!

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Topics: eye health

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