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WHAT DOES 20/20 OR 20/40 VISION MEAN?

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The 20/20 nomenclature is an old system devised by Hermann Snellen, a Dutch Ophthalmologist, in 1862. 20/20 means the viewer can see at 20 feet what a "normal" person would see at 20 feet. 20/40 means the viewer sees at 20 feet what the "normal" person can see at 40 feet, i.e. the viewer can't see quite as well as "normal". This system is somewhat misleading. 20/40 vision is not twice as bad as 20/20 vision. In fact, most people with 20/40 or better vision can, and usually do, function without wearing correction. 20/40 vision or better is required to pass a drivers test in Colorado. After LASIK, 97% of patients have 20/40 or better vision and over 80% have 20/25 or better vision. In the "real world" the difference between 20/20 and 20/25 vision is negligible. Interestingly, Snellen used test subjects to determine what people with "perfect" vision could see, and called that level of vision "20/20". It turns out that his test subjects were slightly nearsighted and, therefore, people ... more
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