What is visual acuity?

Visual acuity is measured using the Snellen scale. A Snellen test usually consists of a number of rows of letters which get smaller as you read down the chart.

On the Snellen scale, normal visual acuity is called 6 / 6, which corresponds to the bottom or second bottom line of the chart. If you can only read the top line of the chart then this would be written as 6 / 60. This means you can see at 6 metres what someone with standard vision could see from 60 metres away.

The figures 6 / 60 or 3 / 60 are how the result of a Snellen test are written. The first number given is the distance in metres from the chart you sit when you read it. Usually this is a 6 (for 6 metres) but would be 3 (for 3 metres) if you were to sit closer to the chart.

The second number corresponds to the number of lines that you are able to read on the chart. The biggest letters, on the top line, correspond to 60. As you read down the chart, this number gets smaller as it corresponds to the lines with smaller letters. Someone with standard vision can read towards the bottom of the chart. Standard vision can be referred to 6 / 6 vision.

For example, if the second line of the chart was marked as the 36 line, a person with standard vision (6 / 6) would be able to read this line on the chart when it was 36 metres away. However, if you had a Snellen score of 6 / 36, you would only be able to read the same line at 6 metres away. In other words you need to be much closer to the chart to be able to read it. Generally the larger the second number is, the worse your sight is.

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