Doctor's Answers (2)
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.
Vision is usually recorded by having a person read a chart of various letters, which vary from being large at the top to small at the bottom.
Near the bottom of the chart, there is a line of letters called the 20/20 or 6/6 line. More precisely, each element of a letter on this line (eg the width of the vertical part of the letter T) subtends an angle of a minute of arc at the nodal point of the eye.
20/20 or 6/6 vision merely means someone can read the letters on this line of the chart.
It can also be taken to mean that what a person with 'normal' vision can see at 20 feet (6 meters), the person being tested can also see at the same 20 feet (6 meters). In other words, 20/20 or 6/6 vision means 'normal vision'.
Note that testing vision in this way can be done with or without glasses.
Therefore 20/20 without glasses could be commonly considered 'perfect vision/eyesight'. On the other hand, 20/20 with glasses means that vision is normal, but only while wearing glasses-ie that there is significant focusing issues/refractive error affecting vision, but that once that is corrected with lenses the small letters can be seen as well.
Some people may also be able to make out the 20/20 letters even though they have slight blurring or a very small amount of refractive error/aberrations. In other words, someone's 20/20 may be a bit different from someone else's 20/20. Also, some people can see even better than 20/20, eg 20/15 or 20/10. So is it possible to be better than perfect? That depends on a person's definition of perfect.
As you can see, when specifying someone's vision/visual acuity, a few extra things (besides the Snellen number) need to be mentioned to really know how good someone's vision is. But at the most basic level, 20/20 means pretty good vision.