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Ask Marilyn: More 20/20 Vision Explanation
Jo Conti of Tucson, Arizona, writes:
Marilyn: Is it possible that in your column about 20/20 vision, you meant to say that 20/30 vision means that you need to stand at 30 feet in order to see what is normally viewed at 20 feet? (October 7, 2012) Instead of the other way around? If one sees at 30 feet what ordinary people see at 20 feet, isn't one's vision worse, not better than 20/20? Please clarify this.
Other readers asked similar questions, and no wonder. I think that Snellen fractions are inherently confusing, and I believe the concept should be replaced. Anyway, you write, "If one sees at 30 feet what ordinary people see at 20 feet..." For the top number in the Snellen fraction, you always stands at 20 feet, not 30 feet. That's the standard Snellen chose.
So when you stand 20 feet from the eye chart, and you see clearly what eyes normally see at 20 feet, your vision is called normal. That's 20/20 vision. But when you stand 20 feet from the chart and can't see clearly what normal eyes see at that distance, your vision is less sharp. When you stand 20 feet from the chart and can see what normal eyes can see clearly at, say, 30 feet, your vision is 20/30. That's not as good as 20/20 vision.
But let's go back to your example, "If one sees at 30 feet what ordinary people see at 20 feet..." If this were indeed the case (forget Snellen fractions for a moment), your vision would be better then normal, not worse. You can see an object clearly at 30 feet, but ordinary people can't. They must move closer—up to 20 feet—before they can see it in focus.