We usually measure vision by getting a person to read down a letter chart from top to bottom with decreasing sizes. Each line of letters is denoted by a number. 20/20 means one could see clearly at 20 feet what should normally be seen at that distance.
It is a term used to indicate normal visual acuity measured at 20 feet. If you have 20/40 vision, it means that you have to be as close as 20 feet to see what a person with normal vision can see at 40 feet. Note that the nominator is simply the distance for measurement, while the denominator represents the size of the target on a calibrated letter chart.
Visual acuity only indicates the clarity of vision at a designated distance. Having a 20/20 vision does not necessarily imply perfect eyesight as there are other visual aspects including contrast sensitivity, visual field function, eye coordination, depth perception, colour vision, etc.
For instance, a person with hyperopia (far-sightedness) may see 20/20 or even better at distance but not so for near tasks. Another person with cataract may still be able to see 20/20 but with an impaired contrast sensitivity.
Therefore, a more detailed and comprehensive eye examination would be required before we can draw any conclusions.
