i read a mesage on an eye chart that said something about viewing the chart at an angle of one minute (1/60th degrees). that makes no sense because of the infinitesimal size of one minute at a distance of a few feet.
2007-02-19 14:36:29 · 3 answers · asked by Anonymous in Science & Mathematics ➔ Physics
Snellen defined “standard vision” as the ability to recognize one of his optotypes when it subtended 5 minutes of arc, thus the optotype can only be recognized if the person viewing it can discriminate a spatial pattern separated by a visual angle of 1 minute of arc.
In the most familiar acuity test, a Snellen chart is placed at a standard distance, twenty feet in countries where that is the customary unit of measure. At this distance, the symbols on the line representing "normal" acuity subtend an angle of five minutes of arc, and the thickness of the lines and of the spaces between the lines subtends one minute of arc. This line, designated 20/20, is the smallest line that a person with normal acuity can read at a distance of twenty feet.
It other words, the person can see separate lines that are separated by 0.070 inch, at a viewing distance of 20 feet.
2007-02-19 14:55:24 · answer #1 · answered by morningfoxnorth 6 · 0⤊ 0⤋
Hi. If you actually figure the tolerance for the head position at 20 feet then it is not that nonsensical.
2007-02-19 14:52:58 · answer #2 · answered by Cirric 7 · 0⤊ 0⤋
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2016-12-17 14:15:03 · answer #3 · answered by hayakawa 4 · 0⤊ 0⤋