Changes

==Origin==

==Origin==

[https://nordan.daynal.org/wiki/index.php?title=English#ca._1100-1500_.09THE_MIDDLE_ENGLISH_PERIOD Middle English] ''acuite'' acridity, from Middle French ''acuité'', from Medieval Latin ''acuitat''-, ''acuitas'', from [[Latin]] ''acuere''

[https://nordan.daynal.org/wiki/index.php?title=English#ca._1100-1500_.09THE_MIDDLE_ENGLISH_PERIOD Middle English] ''acuite'' acridity, from Middle French ''acuité'', from Medieval Latin ''acuitat''-, ''acuitas'', from [[Latin]] ''acuere''

*[http://en.wikipedia.org/wiki/16th_century 1543]

*[https://en.wikipedia.org/wiki/16th_century 1543]

==Definition==

==Definition==

*1: keenness of [[perception]]

*1: keenness of [[perception]]

[[Normal]] visual acuity is commonly referred to as ''20/20 vision'', the metric equivalent of which is 6/6 vision. At 20 feet or 6 metres, a human eye with nominal performance is able to separate lines that are one arc minute apart (equivalent to lines that are spaced 1.75 mm apart). A vision of 20/40 is considered half as good as nominal performance. A vision of 20/10 is considered twice as good as nominal performance.

[[Normal]] visual acuity is commonly referred to as ''20/20 vision'', the metric equivalent of which is 6/6 vision. At 20 feet or 6 metres, a human eye with nominal performance is able to separate lines that are one arc minute apart (equivalent to lines that are spaced 1.75 mm apart). A vision of 20/40 is considered half as good as nominal performance. A vision of 20/10 is considered twice as good as nominal performance.

In an [http://en.wikipedia.org/wiki/Eye_exam eye exam], [[lenses]] of varying powers are used to precisely correct for [http://en.wikipedia.org/wiki/Refractive_error refractive errors]. [[English]] [[alphabet]] letters are typically used (as in the classic [http://en.wikipedia.org/wiki/Snellen_chart Snellen chart]) as most people will recognise them but other [[symbols]] (such as a letter E facing in different directions) are also used.

In an [https://en.wikipedia.org/wiki/Eye_exam eye exam], [[lenses]] of varying powers are used to precisely correct for [https://en.wikipedia.org/wiki/Refractive_error refractive errors]. [[English]] [[alphabet]] letters are typically used (as in the classic [https://en.wikipedia.org/wiki/Snellen_chart Snellen chart]) as most people will recognise them but other [[symbols]] (such as a letter E facing in different directions) are also used.

In the expression, 20/40 vision, the 20 is the distance in feet between the subject and the chart. The 40 means that the subject can read the chart (from 20 feet away) as well as a normal person could read the same chart from 40 feet away. This is calculated by finding the smallest [http://en.wikipedia.org/wiki/Eye_chart optotype] they can identify and calculating the distance at which it has a [http://en.wikipedia.org/wiki/Visual_anglev isual angle] of 5 [http://en.wikipedia.org/wiki/Minute_of_arc arcminutes].

In the expression, 20/40 vision, the 20 is the distance in feet between the subject and the chart. The 40 means that the subject can read the chart (from 20 feet away) as well as a normal person could read the same chart from 40 feet away. This is calculated by finding the smallest [https://en.wikipedia.org/wiki/Eye_chart optotype] they can identify and calculating the distance at which it has a [https://en.wikipedia.org/wiki/Visual_anglev isual angle] of 5 [https://en.wikipedia.org/wiki/Minute_of_arc arcminutes].

A person who can correctly identify letters on the lowest line of the Snellen chart is able to [[discern]] individual lines that are [[separated]] by a visual angle of one arc minute.

A person who can correctly identify letters on the lowest line of the Snellen chart is able to [[discern]] individual lines that are [[separated]] by a visual angle of one arc minute.

The 20/x number does not directly relate to the [http://en.wikipedia.org/wiki/Eyeglass_prescription eyeglass prescription] required to correct vision, because it does not specify the [[nature]] of the [[problem]] corrected by the lens, only the resulting performance. Instead an eye exam seeks to find the prescription that will provide the best corrected visual [[performance]] achievable. This may be greater or lesser than 6/6 for many reasons. In other words, 20/20 vision or 6/6 vision does not necessarily correspond to the best possible visual acuity a subject may [[achieve]], but once this standard is attained the subject is considered to have achieved "[[normal]]" visual acuity. Since Jan 2013 a visual acuity of 20/5 is considered the best possible vision.[http://en.wikipedia.org/wiki/Visual_acuity]

The 20/x number does not directly relate to the [https://en.wikipedia.org/wiki/Eyeglass_prescription eyeglass prescription] required to correct vision, because it does not specify the [[nature]] of the [[problem]] corrected by the lens, only the resulting performance. Instead an eye exam seeks to find the prescription that will provide the best corrected visual [[performance]] achievable. This may be greater or lesser than 6/6 for many reasons. In other words, 20/20 vision or 6/6 vision does not necessarily correspond to the best possible visual acuity a subject may [[achieve]], but once this standard is attained the subject is considered to have achieved "[[normal]]" visual acuity. Since Jan 2013 a visual acuity of 20/5 is considered the best possible vision.[https://en.wikipedia.org/wiki/Visual_acuity]

There exists also a reference to ''tactile acuity'' regarded as the [[ability]] to [[resolve]] fine spatial details of an object with the [[sense]] of [[touch]].

There exists also a reference to ''tactile acuity'' regarded as the [[ability]] to [[resolve]] fine spatial details of an object with the [[sense]] of [[touch]].