Visual Acuity: How it Measures How Well You See Things


Visual acuity is a term that is used to describe how well we see, or how “sharp” our vision is.  The testing of visual acuity was thought to begin in the 1800’s when doctors were looking for a way to standardize the measurement of how well a person sees objects. This standardization was historically important because back in the 1800’s, it was commonly thought that people’s eyes all changed at the same rate based on age!  Throughout the standardization process, doctors discovered that this rate of change was actually different for everyone, and realized that they could develop a test to demonstrate and document each individual’s eyesight.


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The application for the visual acuity test, using letters or symbols, began to grow rapidly and soon became the preferred method for testing patient’s vision.  Most of us have heard of the term, “20/20 vision”.  20/20 is the visual acuity that the human eye should be able to see.  How well our eyes should be able to see is based upon the anatomy of the eye and takes into account many complex measurements.  It is interesting to note that scientists can estimate the visual acuity of different animals based on the size and shape of their eyes.  Eagles and other birds of prey, for example, are thought to see 20/5 and 20/2, much clearer than 20/20.

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Why the term 20/20?  

It is, again, based on the anatomy of the eye.  Visual acuity measures our central vision, which is where our vision is the clearest.  Think of a target with a bulls-eye in the middle; we are measuring the vision in the bullseye. 20/20 comes from the optimal testing distance being 20 ft (or 6 meters in countries that use the metric system) and seeing the lowest letters on the eye chart (a “20” size letter).  The letters are given a unique numerical value and are based on the eye’s ability to see clearly.  For example, within the United States, an individual is “legally blind” if their best visual acuity including the use of contact lenses or eyeglasses is 20/200 or worse. This would mean that you may be able to read the big “E” at the top of the chart, but would not be able to read letters any lower than that. 20/200 vision indicates that the patient can read a letter at 20 feet that individuals with “normal” vision would be able to recognize from 200 feet. Most states require a persona to have a minimum visual acuity of 20/40 in order to obtain and renew a driver’s license. At the same time, “superhuman vision” is also possible at 20/10 and even 20/8, although these cases are extremely rare.

Measuring how well we see is important for many reasons.  Not seeing well can mean more than just needing glasses– Some may never be able to see 20/20, even with correction such as glasses and contacts, due to health issues. Many eye diseases affect how well we see and monitoring our vision with the EyeQue Insight is a great way to track changes in our acuity.  Driving, sports performance and seeing the board at school are daily activities that require excellent vision.  If you are having difficulty in one of these areas, check your acuity with the Insight and get a comprehensive exam with your eye doctor.  You will be glad when you can see better!


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