Snellen chart

This is the eye chart used by American eye doctors for measuring vision.  The letters on this eye chart are calibrated based on distance measurements, comparing your vision to someone with “perfect” eyes.  For example, if you have a vision of 20/60 … this means that the letters you can barely see when standing 20 feet from the chart can be easily seen by a “perfect-eyed person” standing a full 60 feet away from the same chart!  In the USA and many European countries, legal driving vision is 20/50 and you are considered legally blind if your corrected vision is 20/200 or worse.  The Snellen chart is calibrated to be 20 feet from the exam chair you are sitting in … but few eye doctors have exam rooms this long.  To offset this, mirrors are used to obtain the full 20-foot distance.