We see things clearly when our eye is able to distinguish two objects (2 keys) or small details of an object (cricket ball & its seams).
And our eye can distinguish these only till a distance, beyond which things become blurry.
But the distance till which you can distinguish objects is not constant and it depends upon the size of the objects and the distance between the two.
E.g. if two keys are 1 cm apart, you will be able to see them as two for a few metres; if they were 50 cm apart, you can distinguish them for a longer distance.
The distance between two objects forms an angle for the human eye (Image A above).
The farther things go from you, the smaller the angle for the human eye becomes (Image B).
Nature has given a ‘limit’ (0.02 degrees) as to how small this angle can become before the human eye can’t see things clearly.
From this limit, science tells us that at a distance of 1 km, someone with “normal distance vision” can see an object roughly between 30cm to 60cm in diameter (depending on light, air quality etc.).
To check human eyesight a chart, Snellen chart (Image C), is used that displays letters of progressively smaller size.
20/20 vision means that the person being tested sees the same line of letters at 20 feet that person with “normal distance vision” sees at 20 feet.
20/40 vision would mean that the test subject sees at 20 feet what a person with normal vision sees at 40 feet.
20/20 is imperial measurement and 6/6 is metric measurement (6 metres = 19.68 feet); so both of them are used interchangeably.
20/20 just indicates sharpness & clarity of vision ‘at a distance’; there is plenty that an eye needs to achieve before it could be said to have perfect vision.
