Many people will say with pride, “Yeah, I’ve got 20/20 vision”. But, what even is that? What do those numbers mean?
The overall figure is your “Visual Acuity Grade”. And the first number is always 20, because the 20 refers to the distance from which you’re supposed to be sitting or standing from the eye chart. And that distance isn’t random: rays of light are just about parallel when twenty feet from the eye chart, so that the muscle controlling the shape of the lens in a normal eye can be in a state of relative rest when viewing the chart.
The second number represents the distance at which a normal eye should be able to see the letters on that particular line. The third from the bottom line on most eye charts is the 20/20 line, so if you can see the letters on that line, you have 20/20, or “normal”, vision. A higher second number indicates that your vision is subnormal. For instance, if you have 20/50 vision, you’re able to discern letters that “normal” observers could see from fifty feet away. If you were able to achieve the highest possible score on the visual acuity test, which is 20/10, that means you can spot letters that a normal person could only detect if he or she were 50% closer to the chart, or at 10 feet away.
But, what if you miss a letter on one of the lines? Would you still get credit for the whole line? The answer is “yes’. You just have to be able to identify a majority of the letters on a line to get full credit for reading that line. Ah…if only our high school teachers graded as easily as our eye doctors…
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