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What does 20/20 mean?
In the term 20/20 vision, the first number is referring to the distance between the person that is being tested and the eye chart (which is always 20 feet away). The second number represents the distance that the average person can see the eye chart. A person with 20/20 vision can read a specific "normal" size letter when it is 20 feet away.
A person that has 20/40 vision can only read letters that a person with 20/20 vision could read from 40 feet away, or would require the letters that are twice the size of the 20/20 letters on the eye chart.
Finally, someone that has 20/400 vision sees the chart the way a person that has 20/20 vision and is standing 400 feet from the chart.
What is a refractive error?
There are four types of refractive errors: myopia (nearsightedness), hyperopia (farsightedness), astigmatism (uneven focusing power), and presbyopia (age-related inability to focus up close). In normal vision, light rays travel through the eye and focus on the retina in the back of the eye. As these rays pass through the eye they are "refracted" or bent. If rays coming from a distant object reach the retina in sharp focus, you will see clearly, but if they are not bent the precise amount, vision will be out of focus. In other words, a refractive error means that the shape of your eye doesn't refract the light properly, so that the image you see is blurred. Those people with long eyeballs tend to be nearsighted and those people with short eyeballs tend to be farsighted.
How are refractive errors corrected?
You should discuss your refractive errors and your lifestyle with your ophthalmologist to decide on which correction will be most effective for you.
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