Visual Acuity

The eye's visual acuity, or ability to resolve fine detail and form sharp images is determined by a standard test.  The classic chart that is used is called the Snellen Chart.  It is based on the eye's ability to resolve an angular subtense of 1 arc minute (1/60 of a degree).  The test is standardized to be given at a distance of 20 feet, with the letters scaled accordingly.  If you can clearly see the "line" that subtends 1 arc minute, you have "20/20" vision.  If you can only see letters that are say twice as large (2 arc minutes in size) then you have "20/40" vision.  This means that the letters you can see at 20 feet can be read by a "normal" eye at 40 feet.  The following picture shows one version of a Snellen Chart: