I can help you see 20/20, but I can’t make you see the future.

Do you have 20/20 vision?  More importantly, do you know what 20/20 vision means?  A lot of optical chain stores tout 20/20 vision as the be-all end-all of visual greatness; like, if you had anything less than 20/20 vision you might as well be a leper.  Truth is, in the grand scheme of things, 20/20 isn’t all that spectacular.  Ask an eagle; they can see along the lines of 20/4 or better.  On second thought, you’d probably do better to NOT ask an eagle; it probably wouldn’t understand the question and might even try to steal your purse.

If you can read this, you can read this.

20/20 is a measure of visual acuity, which is a fancy way of saying that it’s the smallest possible detail that your eyes can discern at a certain distance.  Obviously, the closer you get to an object, the easier it is to make out the details.  That’s why we use a standard distance for measuring visual acuity, and that distance is 20 feet, unless you’re reading this from England where they use 6 meters (also known as metres, not to be confused with meteors).  That is, if you have 20/20 vision in the U.S., you have 6/6 vision in the U.K.  Or if you’re from Hell, you have 666 vision.

Since 20 feet is the standard testing distance used, that number becomes the numerator of the fraction, or the top number for those of you who failed fourth grade.  This next part is where it gets a little tricky.  A 20/20-sized letter on a visual acuity chart is defined as a letter that a person with “normal” or “average” vision should be able to discern at a distance of 20 feet.

So let’s say that you can’t make out if that letter on the 20/20 line is a T or an A.  Your optometrist would then show you the next line up, which is just a bit larger; this is the 20/25 line.  If you can make out each letter on the 20/25 line but none on the 20/20 line, that means you have 20/25 vision.  That also means that if a person with 20/20 vision wanted to know what you see, they would have to stand at a distance of 25 feet from whatever the target is; in other words, a person with 20/20 vision can see details at a farther distance than a person with 20/25 vision.

"My vision is so good it can melt stop signs."

Another important aspect here is the idea of best-corrected visual acuity.  There are essentially two ways of checking your vision, with your glasses or contacts and without your glasses or contacts (if you wear such things).  Your vision with your glasses or contacts is called your best-corrected visual acuity, or BCVA.  The difference between your BCVA and your visual acuity without glasses can be enormous; to use myself as an example, my uncorrected visual acuity is about 20/400, which means I can barely discern the “big E” without my glasses.  However, my BCVA is 20/20.

One more thing to note:  20 feet was chosen as a testing distance because it “approximates optical infinity.”  This is what I was taught in optometry school, but it seems like a clunky phrase and it’s almost impossible to explain such a concept to a patient who is sitting in your chair.  Often the best I can do is profess that I’m not a miracle worker, and that the best I can do is make you see 20/20.   I suppose we could arrange for an ocular transplant with an eagle.

I’m pretty sure there’s a guy in a van in the alley next door where I can refer you for that procedure.

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One response to “I can help you see 20/20, but I can’t make you see the future.

  1. Pingback: This Blog Will Cure Your Blindness | Apertis Oculis

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