Random Thought #71

I don't know what prompted me to think of this but for whatever reason, I was thinking about vision and what 20/20 vision meant. I know it means you have normal good vision, but I wondered what the numbers actually meant. I found out that it represents visual acuity and that the first number is always the same. It represents the distance from the eye chart that someone stands when trying to make out the various lines of letters of different sizes. Its 20 ft. The second number is a little more complicated. According to wikipedia, it is "the size of the letters, specifically it denotes the separation at which the lines that make up those letters would be separated by a visual angle of 1 arc minute, which for the lowest line that is read by an eye with no refractive error (or the errors corrected) is usually 20 feet (6.1 m)". So the chart is designed so that the size of the letters on the last row if there were multiple lines, the space between those lines would be 1 arc minute, or 1/60th of 1 degree. Since you are reading the chart from 20 feet away, if you can read that bottom line, then the denominator becomes 20 and you have 20/20 or normal vision. If however, you had 20/10 vision, then that means you can see the same level of detail from 20 feet as someone with 20/20 vision could see at 10 feet and vice versa. If you had 20/40 vision, then you could see the same level of detail at 20 feet as someone with normal vision could see at 40 feet. I thought it was kind of interesting that they chose 20 feet. I also thought that it was interesting that everywhere else, normal vision is 6/6 because 20 feet is roughly the same as 6 meters. Bet you've never heard of 6/6 vision as being normal before huh?