We have all heard someone brag about their 20/20 vision (or maybe it was even us), but what does that term really mean?  To understand the basics of the “20/20” claim, it might help to get a bit more familiar with the Snellen vision test chart – see fig 1, below.

The Snellen eye chart test for visual acuity.

The Snellen eye chart contains several lines of block letters, the largest, a single letter, sits on the top row. The letters on each successive row below decreases in size.  The standard Snellen eye chart only typically uses only the following letters: C, D, E, F, L, O, P, T, and Z.

Most of us are familiar with having used the Snellen chart whenever we see an ophthalmologist or optometrist but when you look a little closer at the area bordered by the green line (for sake of convenience), then the picture becomes somewhat clearer.  This is the “20/20” area of the chart and it means that the average person viewing this chart from twenty feet will be able to read this line without difficulty. Interestingly, outside of the United States the reading is 6/6 as the distance is measured in meters instead of feet.  This reading, be it 20/20, 6/6 or some other number is an expression of Visual Acuity. And it references the smallest letters accurately read on the chart. 

Visual acuity in perspective.

Interpreting visual acuity is based on a very simple rule,  if a person can recognize the line of letters twice as large as the reference line (i.e the “20/20” line) then that person’s magnification requirement (lens correction) is said to be 2X; and so their acuity measurement would be written as follows:  Reference line ( this is our 20/20 line) = “1” and the magnification requirement = ‘2’ for a reading of 1/2, therefore 20/20 multiplied by ½ = 20/40.

Another example is if a person receives a magnification requirement of 10.  Then our reading becomes 1/10, and 1/10 multiplied by 20/20 = 20/200 which, incidentally, is where legal blindness begins.

20/20 the holy grail of vision.

Well, it’s not all it’s cracked up to be.  Unfortunately, having 20/20 vision does not necessarily mean that one has any special superpower as far as eyesight is concerned.  What 20/20 means is that the individual has good eyesight that is more or less statistically average.

Find a professional for eye exams.Like many other useful tools these days, the Snellen eye chart can be found online and downloaded.  However, caution is advised. In order to accurately determine the quality of your vision and the overall health of your eyes, it is crucial that you consult a licensed professional like Dr. Dolezal at Innovations in Eyecare.  Attempting to diagnose your visual acuity in your own home can be a fun exercise but your eye health professional has the standards and techniques to help ensure you maintain not simply high visual acuity but healthy eyesight in general.