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What's in a number?
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20/20 is a number
that represents the average person's visual acuity (the clarity
or sharpness of vision). This is what doctors regard as "normal"
or standard vision which the majority of the population has. If
you have 20/20 vision then you have normal vision. This is measured
with a Snellen chart which are lines of text that get smaller and
smaller. If at 20 feet away you can see the 20/20 line on a Snellen
chart you have 20/20 vision (See diagram below). In Europe, the
metric system is used so you have 6/6. Meaning 6 meters instead
of feet, but the principle is exactly the same. |
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| If a person has 20/40 vision, they see the same thing at 20 feet away that a person with 20/20 vision can see at 40 feet away. Or another way to put it is that a person with 20/20 vision has vision twice as good as someone with 20/40 vision. (See diagram below) | |||||||||||
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| The Snellen chart was invented in 1862. It's important to note that 20/20 vision does not mean perfect vision. It only indicates the sharpness or clarity of vision at a distance. There are many other important aspects of vision such as peripheral vision (side vision), eye coordination, depth perception, focusing ability and color vision which all contribute to overall vision. Ghosting, glare, halos and other vision problems which many people with Keratoconus have are not measured with a Snellen chart. | |||||||||||
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A diopter is a unit of measurement that indicates the amount of correction needed to change your vision to be as close to 20/20 as possible. The greater the diopter, the greater the error and hence the more correction required. This measurement is normally used by doctors for your lenses or eye glass prescription. |
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How do I read my prescription? An eye glass
prescription has 3 numbers. Useful links |
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