20/20 vision

What is 20/20 vision ? 20/20 (or 6/6 in the UK) is the Snellen fraction indicating the distance at which the test is done / the distance a letter can be identified.  Simply put, the first number is the testing distance (USA 20 feet, UK 6 meters) and the second number indicates the letter size. The smaller the number the smaller the size.

This refers to our ability to see fine detail under ideal lighting conditions (high contrasting characters).  The best that we can see is 6/3.2 and this is limited by the density of photo receptors in the fovea (the part of our retina that is used to seee fine detail) and the clarity of our Cornea, lens and gel inside the eye. The latest laser operations to improve vision by altering the shape of the cornea to try to reach the 6/3.5.  Tiger Woods, the famous American golfer and one of the richest sports men in the world had his vision improved temporarily to 6/4.

So where did the notion that 20/20 is perfect vision come from ? It is believed that this is a legacy from the first world war where conscripted men were given a rudimentary test and told that if they could see the 20/20 line, then they had perfect vision. i.e. good enough to be sent to the front as cannon fodder.

Actually 20/20 is just a mathematical nicety. It is the ability to resolve a difference of an angle of 1 minute of arc (1/60th of a degree).  This is typically done using a capital letter E where each arm and gap is 1’ of arc and the letter is sized for a distance of 20 feet. This has nothing to do with what we should be able to see and that is why there are lines below the 20/20 line.

 Lets see what its like.  A test chart that optometrists use is the best place to start. The line with red markers either side is the  6/6 line.  Most people should be able to see below this line! Please note that this is a very simple explanation and there are many other aspects to seeing well.

From 6 metres this is what a snellen chart looks like to a person with 6/4 vision

This is what a person with good / normal vision would be likely to see.

Driving decisions can be made early -  “Waterworks, I’ll need to take the next exit”

This is roughly the legal limit

Is this Waterworks Corner ? 
Do I move over just incase ?.

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