Editing Visual acuity (section)

== Definition ==
[[File:Eye examination visual acuity.jpg|thumb|alt=An elderly woman is undergoing an eye examination at a hospital|Eye examination for visual acuity]]
[[File:ETDRS Chart R.svg|thumb|The [[LogMAR chart|LogMAR – ETDRS Chart]], designed in 1976, to test for visual acuity]]

Visual acuity is a measure of the spatial resolution of the visual processing system. VA, as it is sometimes referred to by optical professionals, is tested by requiring the person whose vision is being tested to identify so-called optotypes – stylized letters, [[Landolt C|Landolt rings]], [[Lea test|pediatric symbols]], [[E chart|symbols for the illiterate]], standardized Cyrillic letters in the [[Golovin–Sivtsev table]], or other patterns – on a printed chart (or some other means) from a set viewing distance. Optotypes are represented as black symbols against a white background (i.e. at maximum [[contrast (vision)|contrast]]). The distance between the person's eyes and the testing chart is set so as to approximate "[[Infinity focus|optical infinity]]" in the way the [[lens (anatomy)|lens]] attempts to focus (far acuity), or at a defined reading distance (near acuity).

A reference value above which visual acuity is considered normal is called 6/6 vision, the [[United States customary units|USC]] equivalent of which is 20/20 vision: At 6 metres or 20 feet, a human eye with that performance is able to separate contours that are approximately 1.75&nbsp;mm apart.<ref name="Britannica-2008">{{Cite encyclopedia |year=2008 |title=eye, human |encyclopedia=[[Encyclopædia Britannica 2006 Ultimate Reference Suite DVD]]}}</ref> Vision of 6/12 corresponds to lower performance, while vision of 6/3 to better performance. Normal individuals have an acuity of 6/4 or better (depending on age and other factors).

In the expression 6/x vision, the [[Fraction (mathematics)|numerator]] (6) is the distance in metres between the subject and the chart and the [[Fraction (mathematics)|denominator]] (x) the distance at which a person with 6/6 acuity would discern the same optotype. Thus, 6/12 means that a person with 6/6 vision would discern the same optotype from 12 metres away (i.e. at twice the distance). This is equivalent to saying that with 6/12 vision, the person possesses half the spatial resolution and needs twice the size to discern the optotype.

A simple and efficient way to state acuity is by converting the fraction to a decimal: 6/6 then corresponds to an acuity (or a Visus) of 1.0 (see [[#Expression|''Expression'']] below), while 6/3 corresponds to 2.0, which is often attained by well-corrected healthy young subjects with [[binocular vision]]. Stating acuity as a decimal number is the standard in European countries, as required by the [[European Committee for Standardization|European norm]] (EN ISO 8596, previously [[DIN]] 58220).

The precise distance at which acuity is measured is not important as long as it is sufficiently far away and the size of the optotype on the retina is the same. That size is specified as a [[visual angle]], which is the angle, at the eye, under which the optotype appears. For 6/6 = 1.0 acuity, the size of a letter on the [[Snellen chart]] or [[Landolt C]] chart is a visual angle of 5 arc minutes (1 arc min = 1/60 of a degree), which is a 43 point font at 20 feet.<ref>Table 3, page 20, {{Cite web |last=Morrison |first=James Barbour , Joanna Zander |date=April 2008 |title=Determining the appropriate font size, and use of colour and contrast for underwater displays - for Defence R & D Canada |url=https://www.researchgate.net/publication/253433795 |access-date=2023-12-20 |website=researchgate}}</ref> By the design of a typical optotype (like a [[Snellen chart|Snellen E]] or a Landolt C), the critical gap that needs to be resolved is 1/5 this value, i.e., 1 arc min. The latter is the value used in the [[Landolt C|international definition]] of visual acuity:

{{in5}}{{math|acuity {{=}} {{sfrac|1|gap size [arc min]}}.}}

Acuity is a measure of visual performance and does not relate to the eyeglass prescription required to correct vision. Instead, an [[eye exam]] seeks to find the prescription that will provide the best corrected visual performance achievable. The resulting acuity may be greater or less than 6/6 = 1.0. Indeed, a subject diagnosed as having 6/6 vision will often actually have higher visual acuity because, once this standard is attained, the subject is considered to have normal (in the sense of undisturbed) vision and smaller optotypes are not tested. Subjects with 6/6 vision or "better" (20/15, 20/10, etc.) may still benefit from an eyeglass correction for other problems related to the visual system, such as [[hyperopia]], ocular injuries, or [[presbyopia]].