What is the purpose of a snellen chart?

Answer 1

The Snellen chart is the chart used by opticians to measure the sharpness of your eyesight (or visual acuity). It was developed by Dr Hermann Snellen in 1862. It has several lines of letters which get smaller as you go down the chart. The chart is placed level with the eyes at a distance of 20 feet (in Europe 6 metres). The standard reference eye is said to have 20/20 vision but this is not particularly good, any more than an IQ of 100 is good. If at 20 feet you can only just read letters that a person with "standard" eyes could read at 40 feet then you have 20/40 vision or if at 20 feet you can only just read letters that a standard eye could read at 60 feet then you have 20/60 vision and so on. If you can read the bottom line of very small letters on the Snellen chart your vision is an excellent 20/10. If you can read the third line from the bottom then it is a "standard" 20/20 which most people have fallen to by the age of 60.