Editing Visual acuity (section)

== Expression ==
{| class="wikitable floatright"
|+Visual acuity scales<ref>{{Cite web |title=Contrast sensitivity |url=http://www.lea-test.fi/en/vistests/instruct/contrast/lowsymbo/Snellen.pdf |access-date=21 July 2018 |website=LEA-Test Ltd}}</ref>
! 20&nbsp;ft
! 10&nbsp;ft
! 6&nbsp;m
! 3&nbsp;m
! Decimal
! {{abbr|MAR|Minimum angle of resolution}}
! LogMAR
|-
|20/1000
|10/500
|6/300
|3/150
|0.02
|50
|1.70
|-
|20/800
|10/400
|6/240
|3/120
|0.025
|40
|1.60
|-
|20/600
|10/300
|6/180
|3/90
|0.033
|30
|1.48
|-
|20/500
|10/250
|6/150
|3/75
|0.04
|25
|1.40
|-
|20/400
|10/200
|6/120
|3/60
|0.05
|20
|1.30
|-
|-
|20/300
|10/150
|6/90
|3/45
|0.067
|15
|1.18
|-
|20/250
|10/125
|6/75
|3/37
|0.08
|12.5
|1.10
|-
|20/200
|10/100
|6/60
|3/30
|0.10
|10
|1.00
|-
|20/160
|10/80
|6/48
|3/24
|0.125
|8
|0.90
|-
|20/125
|10/62
|6/38
|3/19
|0.16
|6.25
|0.80
|-
|20/100
|10/50
|6/30
|3/15
|0.20
|5
|0.70
|-
|20/80
|10/40
|6/24
|3/12
|0.25
|4
|0.60
|-
|20/60
|10/30
|6/18
|3/9
|0.33
|3
|0.48
|-
|20/50
|10/25
|6/15
|3/7.5
|0.40
|2.5
|0.40
|-
|20/40
|10/20
|6/12
|3/6
|0.50
|2
|0.30
|-
|20/30
|10/15
|6/9
|3/4.5
|0.67
|1.5
|0.18
|-
|20/25
|10/12
|6/7.5
|3/4
|0.80
|1.25
|0.10
|-
|20/20
|10/10
|6/6
|3/3
|1.00
|1
|0.00
|-
|20/16
|10/8
|6/4.8
|3/2.4
|1.25
|0.8
| −0.10
|-
|20/12.5
|10/6
|6/3.8
|3/2
|1.60
|0.625
| −0.20
|-
|20/10
|10/5
|6/3
|3/1.5
|2.00
|0.5
| −0.30
|-
|20/8
|10/4
|6/2.4
|3/1.2
|2.50
|0.4
| −0.40
|-
|20/6.6
|10/3.3
|6/2
|3/1
|3.00
|0.333
| −0.48
|}

Visual acuity is often measured according to the size of letters viewed on a [[Snellen chart]] or the size of other symbols, such as [[Landolt C]]s or the [[E Chart]].

In some countries, acuity is expressed as a [[simple fraction]], and in some as a [[decimal]] number. Using the metre as a unit of measurement, (fractional) visual acuity is expressed relative to 6/6. Otherwise, using the foot, visual acuity is expressed relative to 20/20. For all practical purposes, 20/20 vision is equivalent to 6/6. In the decimal system, acuity is defined as the reciprocal value of the size of the gap (measured in arc minutes) of the smallest [[Landolt C]], the orientation of which can be reliably identified. A value of 1.0 is equal to 6/6.

[[LogMAR chart|LogMAR]] is another commonly used scale, expressed as the ([[Common logarithm|decadic]]) logarithm of the minimum angle of resolution (MAR), which is the [[Multiplicative inverse|reciprocal]] of the acuity number. The LogMAR scale converts the geometric sequence of a traditional chart to a linear scale. It measures visual acuity loss: positive values indicate vision loss, while negative values denote normal or better visual acuity. This scale is commonly used clinically and in research because the lines are of equal length and so it forms a continuous scale with equally spaced intervals between points, unlike Snellen charts, which have different numbers of letters on each line.

A visual acuity of 6/6 is frequently described as meaning that a person can see detail from {{convert|6|m|ft}} away the same as a person with "normal" eyesight would see from 6 metres. If a person has a visual acuity of 6/12, they are said to see detail from {{convert|6|m|ft}} away the same as a person with "normal" eyesight would see it from {{convert|12|m|ft}} away.

The definition of 6/6 is somewhat arbitrary, since human eyes typically have higher acuity, as Tscherning writes, "We have found also that the best eyes have a visual acuity which approaches 2, and we can be almost certain that if, with a good illumination, the acuity is only equal to 1, the eye presents defects sufficiently pronounced to be easily established."<ref name="Physiologic Optics">''Physiologic Optics: Dioptrics of the Eye, Functions of the Retina, Ocular Movements and Binocular Vision''</ref> Most observers may have a binocular acuity superior to 6/6; the limit of acuity in the unaided human eye is around 6/3–6/2.4 (20/10–20/8), although 6/3 was the highest score recorded in a study of some US professional athletes.<ref>{{Cite news |last1=Kirschen |first1=David G. |last2=Laby |first2=Daniel M. |name-list-style=vanc |date=1 May 2006 |title=Sports Vision Testing: An Innovative Approach To Increase Revenues |url=http://www.optometricmanagement.com/articleviewer.aspx?articleid=71635 |work=Optometric Management}}</ref> Some [[Bird of prey|birds of prey]], such as [[hawk]]s, are believed to have an acuity of around 20/2;<ref>{{Cite web |last=Kirschbaum |first=Kari |name-list-style=vanc |title=Family Accipitridae |url=http://animaldiversity.ummz.umich.edu/site/accounts/information/Accipitridae.html |access-date=30 January 2010 |website=Animal Diversity Web |publisher=University of Michigan Museum of Zoology}}</ref> in this respect, their vision is much better than human eyesight.

When visual acuity is below the largest optotype on the chart, the reading distance is reduced until the patient can read it. Once the patient is able to read the chart, the letter size and test distance are noted. If the patient is unable to read the chart at any distance, they are tested as follows:

{| class="wikitable floatleft" style="margin:auto;"
|-
!Name
!Abbreviation
!Definition
|-
!Counting Fingers
|CF
|Ability to count fingers at a given distance. This test method is only used after it has been determined that the patient is not able to make out any of the letters, rings, or images on the acuity chart. The letters CF, and the testing distance, would represent the patient's acuity.
''For example, the recording '''CF 5'<nowiki/>''' would mean the patient was able to count the examiner's fingers from a maximum distance of 5 feet directly in front of the examiner.''

(The results of this test, on the same patient, may vary from examiner to examiner. This is due more to the size differences of the various examiners' hands and fingers, than to fluctuating vision.)
|-
!Hand Motion
|HM
|Ability to distinguish whether or not there is movement of the examiner's hand directly in front of the patient's eyes. This test method is only used after a patient shows little or no success with the Counting Fingers test. The letters HM, and the testing distance, would represent the patient's acuity.
''For example, the recording '''HM 2'<nowiki/>''' would mean that the patient was able to distinguish movement of the examiner's hand from a maximum distance of 2&nbsp;feet directly in front of the examiner.''

(The results of the Hand Motion test are often recorded without the testing distance. This is due to the fact that this test is performed after the patient cannot "pass" the Counting Fingers test. At this point, the examiner is usually directly in front of the patient, and it is assumed that the Hand Motion test is performed at a testing distance of 1 foot or less.)
|-
!Light Perception
|LP
|Ability to perceive any light. This test method is used only after a patient shows little or no success with the Hand Motion test. In this test, an examiner shines a [[pen light]] at the patient's pupil and asks the patient to either point to the light source, or describe the direction that the light is coming from (up, out, straight ahead, down and out, etc.). If the patient is able to perceive light, the letters LP are recorded to represent the patient's acuity. If the patient is unable to perceive any light, the letters NLP ('''N'''o '''L'''ight '''P'''erception) are recorded. A patient with no light perception in one eye is considered blind in the respective eye. If NLP is recorded in both eyes, the patient is described as having total blindness.
|}

{{clr}}

=== Legal definitions ===
Various countries have defined statutory limits for poor visual acuity that qualifies as a disability. For example, in Australia, the Social Security Act defines blindness as:

{{blockquote|text=A person meets the criteria for permanent blindness under section 95 of the Social Security Act if the corrected visual acuity is less than 6/60 on the Snellen Scale in both eyes or there is a combination of visual defects resulting in the same degree of permanent visual loss.<ref>''Social Security Act'' 1991 (Cth) {{Cite web |title=Table 13, Schedule 1B |url=https://www.legislation.gov.au/Details/C2011C00765/Html/Volume_3#_Toc304376657}} as at 20 September 2011.</ref>}}

In the US, the relevant federal statute defines blindness as follows:<ref>42 U.S.C. §&nbsp;416(i)(1)(B) (Supp. IV 1986). Cited in {{Cite web |date=9 November 1990 |title=SSR 90-5c: Sections 216(i)(1)(B) and 223(c)(1) and (d)(1)(B) of the Social Security Act (42 U.S.C. 416(i)(1)(B) and 423(c)(1) and (d)(1)(B)) Disability insurance benefits – Interpreting the statutory blindness provision |url=https://www.law.cornell.edu/socsec/rulings/ssr/SSR90-05.html}}</ref>

{{blockquote|text=[T]he term "blindness" means central visual acuity of 20/200 or less in the better eye with the use of a correcting lens. An eye that is accompanied by a limitation in the fields of vision such that the widest diameter of the visual field subtends an angle no greater than 20 degrees shall be considered for purposes in this paragraph as having a central visual acuity of 20/200 or less.}}

A person's visual acuity is registered documenting the following: whether the test was for distant or near vision, the eye(s) evaluated and whether [[corrective lenses]] (i.e. [[glasses]] or [[contact lens]]es) were used:
* Distance from the chart
** D (distant) for the evaluation done at {{convert|20|ft|m|sigfig=1}}.
** N (near) for the evaluation done at {{convert|15.7|in}}.
* Eye evaluated
** OD (Latin ''oculus dexter'') for the right eye.
** OS (Latin ''oculus sinister'') for the left eye.
** OU (Latin ''oculi uterque'') for both eyes.
* Usage of spectacles during the test
** cc (Latin ''cum correctore'') with correctors.
** sc: (Latin ''sine correctore'') without correctors.
* [[Pinhole occluder]]
** The abbreviation PH is followed by the visual acuity as measured with a pinhole occluder, which temporarily corrects for [[refractive error]]s such as myopia or astigmatism.
** PHNI means no improvement of visual acuity using a pinhole occluder.
So, distant visual acuity of 6/10 and 6/8 with pinhole in the right eye will be: DscOD 6/10 PH 6/8. Distant visual acuity of count fingers and 6/17 with pinhole in the left eye will be: DscOS CF PH 6/17. Near visual acuity of 6/8 with pinhole remaining at 6/8 in both eyes with spectacles will be: NccOU 6/8 PH 6/8.

"Dynamic visual acuity" defines the ability of the eye to visually discern fine detail in a moving object.