Visual acuity describes how well you see detail with your central vision.
This is usually measured using a special chart with rows of letters that start with one big one at the top and get smaller row by row - called a Snellen scale.
It is named after Dutch ophthalmologist Herman Snellen who invented it in 1862.
During a routine eye test, the optician or ophthalmologist will sit a person 6 metres from the chart. If glasses or contact lenses are worn, these should be used for the test.
Each eye is tested while the other one is covered. Eyes may also be tested together.
The rows of letters correspond to the minimum size of letter that could be seen by someone with normal vision from 6m up to 60m.
A score on the test is given for each eye.
Each eye is assessed with two numbers, such as having 6/6 vision.
The first number is the distance the chart is viewed from.
The second is the distance at which the person being tested can see a letter clearly.
This used to be in the form 20/20 when the distance was measured in feet not metres.
The result of the Snellen test is just one aspect of vision tests an optician or ophthalmologist may carry out, others include checking reading distance (near vision), peripheral vision or colour vision.
If you can only read the big letter on the top line, that's recorded as 6/60 - you can see at 6m what can usually be seen from 60m with normal vision.
If you can identify 2 letters from the second row, that's 6/36
Three letters from the third row is 6/24
This continues down to the seventh row, where around 7 letters corresponds to 6/6 vision.
Snellen scale test scores for each eye and both - called binocular vision - might be:
|Left eye||Right eye||Binocular vision|
Visual acuity and driving
For driving, the Department of Work and Pensions says a person needs to be able to read the third line up from the bottom of the chart with just their eyes, or with glasses or contacts - so 6/9 vision also sometimes expressed as 0.5.
Standards are higher for lorry drivers and bus drivers.