|MadSci Network: General Biology|
William, You've asked a good question. The number 20/20 is used very often to mean perfect, but not many people really know what they're talking about. First off, I'll get into where the old 20/20 came from, then I'll talk about why 20/20 isn't really perfect vision. The 20/20 designation was invented by Snellen a long time ago (at least a hundred years, anyway). He found an assistant with exceptionally good vision and compared everyone to that assistant. He made up a chart, and at 20 feet of distance, his assistant could just read the line he designated 20. So, 20feet/20size letter, and thus 20/20. If a person with decreased vision could just make out a larger line at 20 feet that his eagle-eyed assistant could read at 40 feet, that person had 20/40 vision. That is, they could read at 20 feet what a person with 'perfect' vision could read at 40, and so, 20/40. Obviously, that's a system that's hard to reproduce. So instead of having every eye care person in Europe find a sharp eyed assistant, the modern Snellen chart was invented with a mathematical basis. The Snellen chart of today is easily recognized by the big "E" on top. Anyway, the 20/20 line on a modern Snellen chart is constructed so that from top to bottom, each letter subtends an angle of 5 minutes at your eye (a minute is 1/60th of a degree). To save you the math, that means a letter on the 20/20 line at 20 feet of distance is about 1/3 of an inch high. For more mathematical reasons we won't venture into, the 20/40 line is exactly twice as big as the 20/20, the 20/60 line is 3 times as large, etc. That big E on top is usually 20/400, hence 20 times as large as the 20/20 letters. So a person with 20/400 vision needs to be 20 times as close as a person with 20/20 vision in order to see the 20/20 line. If you followed the math, though, you realize that a Snellen chart has to be at an exact distance to be correct. Unless you're at 20 feet, those little letters won't be the right height. If your optometrist or ophthalmologist has mirrors at the ends of his/her exam room, it's probably to make the light from the chart travel 20 feet in a 10 foot long room before you see it, so then it'll be the right size. Now on to whether people see better than 20/20. The quick answer is yes, nearly everyone does at some point in their life. For people 35 and under, in fact, eye care practitioners aim for 20/15 with glasses or contacts. Some folks are lucky enough not to need glasses or contacts to see 20/15, but with correction, nearly all people young enough and without eye diseases can do it. A 20/15 letter is 3/4 the size of a 20/20 letter. To put it another way, a 20/15 person can see at 20 feet what a 20/20 person has to move up to 15 feet away to see. The limit on all this is about 20/10, or half the size of that teeny little 20/20 letter. The limits are a combination of the optical aberrations inherent in your eye (it's good, but not perfect) and how tightly your photoreceptors in your retina can be physically packed in. For a real-life example, Mark McGwire the home run king can see about 20/10 with his contacts (poor old Mark can only see 20/400 without them, though). Of course, that brings up the point that there is a lot more to vision than seeing 20/20 (or 20/15). That little letter on the chart isn't moving, and it's brightly lit up, and you're looking straight at it. While driving or playing sports, for instance, your main areas of concern are rarely standing still, brightly lit, and right in the center of your vision all the time. I have a feeling I more than answered your question, but if you have any more questions, let me know. Tom
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