Why 20/20 isn’t perfect-What is visual acuity?

Courtesy of riekhavoc (caughtup?) on Flickr

Courtesy of riekhavoc (caughtup?) on Flickr

Measuring normal vision

When patients come to see me, I need to have a way to compare how they see in relation to someone with normal vision.  So, like most eye doctors, I use a number system called Snellen visual acuity.  This measure of the clarity of vision uses black letters on a white background.  If you have ever had an eye examination, it is when the doctor asks you to read the letters on the chart that starts with a big “E”.  The letters are called optotypes and they have a very specific design that takes into account the size of the lines and the space between them.  While there are other types of visual acuity measurements, this is the most common.

20/20 isn’t perfect

The measure most people want to achieve with Snellen visual acuity is 20/20 vision.  While 20/20 is pretty darn good vision, in reality, 20/20 is not exceptional vision so much as it is more like the lowest possible visual acuity a person can have and be considered normal.  In fact, many people have the potential to see somewhere between 20/16 and 20/12 which means that they see even better than 20/20!

Paul B. (Halifax)

Paul B. (Halifax)

What do the numbers mean?

People also ask me to explain what the numbers mean.  As an example, take a person with 20/40 vision.  A person with 20/40 is at a disadvantage compared to a person with normal, 20/20 vision.  In fact, a person with 20/40 vision would have to stand 20 feet away from something that a person with normal vision can stand 40 feet away from and still see.

A number of factors like eye disease, the eye’s length and curvature, and the quality of connection between the eye and the processing centers in the brain come together to determine visual acuity.  Some factors, like nearsightedness, come with easy solutions like glasses.  Others, like macular degeneration, are much more complex and simple solutions like glasses only offer minimal improvement.  If a patient has very poor visual acuity, they may need magnifiers and telescopic lenses to help.

So,  the next time you go to the optometrist, give the 20/15 line a shot.  Good luck!

~David Ardaya, OD
California Optometric Association
http://eyehelp.org
http://www.coavision.org

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