Page 18 - The UK Guide to Laser Eye Surgery
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18 How the eye works
Understanding visual acuity
What does 20/20 mean?
It is possible to see better than the norm - better than 20/20
‘Visual acuity’ is the standard measure of
a person’s ability to see clearly. When your visual acuity is measured, your optometrist is testing your central vision – your ability to distinguish the details and shapes of objects from a certain distance away.
In the most common visual acuity test,
an optometrist places an eye chart at
a standard distance (twenty feet or six metres, depending on the customary unit of measurement). The patient is then asked to read letters on lines across the eye chart. Towards the bottom of the chart is a line known as the ‘20/20 line’ – this is the smallest line that a person with ‘normal’ visual acuity can read from twenty feet away. Therefore, having ‘20/20 vision’ simply means that you have normal
visual acuity.
Three lines above the normal (20/20) line is the 20/40 line. The letters on the 20/40 line are twice as large as those on the 20/20 line, so a person with normal visual acuity could read them from forty feet away. If the smallest letters that a person can read from twenty feet away are those on the 20/40 line, that person would be said to have 20/40 vision. (This means that the person needs to approach to a distance of twenty feet to read letters that a person with normal acuity could read at forty feet.)
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