The Snellen chart displays letters of progressively smaller size. It was developed by Dutch eye doctor Hermann Snellen in the 1860s.
"Normal" vision is 20/20 and is a term used to define normal visual acuity; the clarity and sharpness of an object at a distance of 20 feet. This means that the test subject sees the same line of letters at 20 feet that a normal person sees at 20 feet. 20/40 vision means that the test subject sees at 20 feet what a normal person sees at 40 feet.
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