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The Snellen Chart provides a standardized test of visual acuity. The chart is placed 6 m - 20 feet - from the subject.

The chart consists of a series of symbols e.g. block letters, in gradually decreasing sizes. The visual acuity is stated as a fraction: the distance from the chart - 6 metres - is the numerator; the distance at which a 'normal eye' would be able to read the last line that the patient is able to read is the denominator. For example, 6/6 vision signifies normal vision i.e. a patient can read a line of symbols at six metres that a person with 'normal visual acuity' would be able to read at six metres.

A person with poor vision may have e.g. 6/10 vision i.e. they are able to read at 6 metres what a person with normal vision can read at 10 metres.

A person with better than normal vision will have a denominator that is less than 6 e.g. 6/5 i.e. a person with this grading of visual acuity can read at six metres what a person with normal visual acuity can only read at 5 metres.