Sunday, October 17, 2010

What does "6/18 or better unaided in each eye correctable to 6/12 or better in each eye using glasses mean?

What does "6/18 or better unaided in each eye correctable to 6/12 or better in each eye using glasses mean?
The first number' 6 ' refers to the distance the patient is from the Snellen eye chart in metres. The line of smallest print on the chart which a person can read at 6 metres in the ' 6 ' line. The next line up is ' 9 ' so a person who can read this with one eye at 6 meters has 6/9 vision in that eye and so on. Normal vision is 6/6 ( or 20/20 referring to charts set in feet). There are also exactly half sized charts read at 3 metres where the lettering is half the size, the resulting reading is the same.
Correctable means that a person with defective vision can be corrected to a certain level only with glasses, i.e. from 6/18 to say 6/9, but other problems do on allow correction to 6/6.
The 6 refers to meters away from the eye chart and the 18 or 12 refers to the number of the line of letters that you can read down to.
Therefore if you can only read down to line 18 without glasses they want you to be able to read down to line twelve with glasses on.
The lines on the chart are not simply labled one two, three etc which I always find confusing.

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