What is a Snellen chart?
by 
wickedwillie
ANSWERS: 2
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The Snellen eye chart is a standardized chart for measuring visual acuity, devised by Dr Hermann Snellen in 1862. The Snellen eye chart has a series of letters or letters and numbers, with the largest at the top. As the person being tested reads down the chart, the letters gradually become smaller. The Snellen fractions, 20/20, 20/30, etc., are measures of sharpness of sight. They relate to the ability to identify a letter of a certain size at a specified distance. "Normal" vision is 20/20. There are two lines smaller than 20/20, 20/15 and 20/10. The first figure is the distance from the chart in feet, the second figure is related to the size of the letters in a specific row of the chart.
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I agree with everything stated in the previous answer. The significant note about the 20/20 letter size is that the details of this target is one minute of arc. Using the E as an example, each arm of the E would be one minute of arc and each gap between each arm would be one minute of arc. So, the overall size of the letter would be five minutes of arc. The snellen chart has worked out the trig for targets held at 20 feet from the observer. When the observer is standing 20 feet away and can identify the letters on the line with a size of 5 minutes of arc, by definition, they are seeing 20/20. Some people with really sharp vision can see 20/15. But 20/15 is just about as good as it gets.
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