What is visual acuity?

Visual acuity (VA) is a measurement of how well you can see detail. It is measured by how far down the letter chart you can read from a specific distance using any glasses or contact lenses you might need. 

Visual acuity is normally tested using the Snellen letter chart at a distance of 6 metres. The visual acuity measurement is written as a fraction. The top number is how far away from the letter chart you are when reading the letters – this is usually 6 (for 6 metres). The bottom number is the smallest line of letters you are able to read on the chart, which also corresponds to the distance that a person with standard vision can read the same line. 

The Snellen chart has lines of letters and the letters reduce in size with each line. Each line on this chart is numbered and the larger the size of the letters, the larger the number. The top line or biggest sized letter on the chart is numbered 60 and the bottom smallest sized line of letters is numbered 6. The number given to each line of letters corresponds to the distance at which a person with standard vision would be able to read that size letter. 

If you have a visual acuity of 6/6, you are able to read to the bottom line of letters at a distance of 6 metres. This is known as having standard visual acuity. 

If a person has a visual acuity of 6/36, it means they can only read down to the line of letters numbered 36 when standing 6 metres from the chart. A person with standard visual acuity (6/6) would be able to read the same line from 36 metres away.  

Generally the larger the bottom number is, the poorer your sight is. If someone is not able to read the top biggest letter at a distance of 6 metres, the chart can be moved forward (usually to 3 metres) to allow them to read it at a closer distance. In this case the visual acuity measurement will have 3 as the top number. 

The saying “20/20 vision” often used when referring to perfect vision, comes from the American method of measurement which uses feet not metres. 20/20 is the same as 6/6. It is possible for people to have vision which is better than 6/6 where the bottom number will be lower than 6 and is measured by them being able to read letters smaller than the 6/6 line.

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