Firstly, normal vision is tested in two main ways. The visual acuity defines the ability to read and detect objects at a distance. It is measured using the vision chart, which everyone is familiar with. The chart has lines of letters in different sizes. The person is then asked to read the letters from top to bottom. The top line has a very large letter, which a normal person would be able to see at 60 metres. However a person with a severe vision disability may only be able to see this at 6 metres. Similarly, for the subsequent lines, a normal person would be able to see the letters at 36, 24, 18, 12, 9 and 6 metres respectively. Normal vision means that a person can read the "6 metre" line (usually the second bottom line) at the correct distance from the chart. This means this person sees at 6 metres what other people with normal vision would see at 6 metres (6/6 vision). This used to be called and is the same as 20/20 vision (6 metres or 20 feet). People with less than normal vision would have their vision described as 6/12 or 6/24 etc. depending on which line they could see on the chart.