Editing Visual acuity (section)

{{short description|Clarity of vision}}
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{{Infobox diagnostic
| name            = Visual acuity
| image           = File:Snellen chart.svg
| alt             = Snellen chart: rows of uppercase letters, the top row contains a very large 'E' and the size of the letters decreases with each row that follows.
| caption         = A typical [[Snellen chart]] that is frequently used for visual far acuity testing
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'''Visual acuity''' ('''VA''') commonly refers to the clarity of [[visual perception|vision]], but technically rates an [[animal]]'s<!-- that includes humans --> ability to recognize small details with precision. Visual acuity depends on optical and neural factors. Optical factors of the [[eye]] influence the sharpness of an image on its [[retina]]. Neural factors include the health and functioning of the retina, of the neural pathways to the brain, and of the interpretative faculty of the brain.<ref>{{Cite book |last1=Cline |first1=D |title=Dictionary of Visual Science |last2=Hofstetter |first2=HW |last3=Griffin |first3=JR. |date=1997 |publisher=Butterworth-Heinemann |isbn=978-0-7506-9895-5 |edition=4th |location=Boston |name-list-style=vanc}}</ref>

The most commonly referred-to visual acuity is ''distance acuity'' or ''far acuity'' (e.g., "20/20 vision"), which describes someone's ability to recognize small details at a far distance. This ability is compromised in people with [[myopia]], also known as short-sightedness or near-sightedness. Another visual acuity is ''[[Near visual acuity|near acuity]]'', which describes someone's ability to recognize small details at a near distance. This ability is compromised in people with [[hyperopia]], also known as long-sightedness or far-sightedness.

A common optical cause of low visual acuity is [[refractive error]] (ametropia): errors in how the light is refracted in the eye. Causes of refractive errors include aberrations in the shape of the [[eye]] or the [[cornea]], and reduced ability of the [[Lens (anatomy)|lens]] to focus light. When the combined refractive power of the cornea and lens is too high for the length of the eye, the retinal image will be in focus in front of the retina and out of focus on the retina, yielding myopia. A similar poorly focused retinal image happens when the combined refractive power of the cornea and lens is too low for the length of the eye except that the focused image is behind the retina, yielding hyperopia. Normal refractive power is referred to as [[emmetropia]]. Other optical causes of low visual acuity include [[astigmatism]], in which contours of a particular orientation are blurred, and more complex corneal irregularities.

Refractive errors can mostly be corrected by optical means (such as [[eyeglasses]], [[contact lenses]], and [[refractive surgery]]). For example, in the case of myopia, the correction is to reduce the power of the eye's refraction by a so-called minus lens.

Neural factors that limit acuity are located in the retina, in the pathways to the brain, or in the brain. Examples of conditions affecting the retina include [[detached retina]] and [[macular degeneration]]. Examples of conditions affecting the brain include [[amblyopia]] (caused by the visual brain not having developed properly in early childhood) and by brain damage, such as from [[traumatic brain injury]] or stroke. When optical factors are corrected for, acuity can be considered a measure of neural functioning.

Visual acuity is typically measured while fixating, i.e. as a measure of central (or [[Fovea centralis|foveal]]) vision, for the reason that it is highest in the very center.<ref>Acuity is highest in a tiny area, sometimes called the "foveal bouquet", with a diameter of only 8{{ndash}}16 minutes of arc (see Strasburger, 2020, p. 10)</ref><ref>{{Cite journal |last=Strasburger |first=H. |year=2020 |title=seven myths on crowding and peripheral vision |journal=i-Perception |volume=11 |issue=2 |pages=1–45 |doi=10.1177/2041669520913052 |pmc=7238452 |pmid=32489576}}</ref> However, acuity in [[peripheral vision]] can be of equal importance in everyday life. Acuity declines towards the periphery first steeply and then more gradually, in an inverse-linear fashion (i.e. the decline follows approximately a [[hyperbolic function|hyperbola]]).<ref name="Strasburger-2011">{{Cite journal |last1=Strasburger |first1=H. |last2=Rentschler |first2=I. |last3=Jüttner |first3=M. |name-list-style=vanc |year=2011 |title=Peripheral vision and pattern recognition: a review |journal=Journal of Vision |volume=11 |issue=5 |pages=13, 1–82 |doi=10.1167/11.5.13 |pmc=11073400 |pmid=22207654 |doi-access=free}}</ref><ref>{{Cite thesis |last=Barghout-Stein |first=Lauren |title=On differences between peripheral and foveal pattern masking |publisher=University of California, Berkeley |year=1999 |name-list-style=vanc}}</ref> The decline is according to ''E''<sub>2</sub>/(''E''<sub>2</sub>+''E''), where ''E'' is eccentricity in [[Visual angle|degrees visual angle]], and ''E''<sub>2</sub> is a constant of approximately 2 degrees.<ref name="Strasburger-2011" /><ref>{{Cite journal |last=Anstis |first=S. M. |year=1974 |title=A chart demonstrating variations in acuity with retinal position |journal=Vision Research |volume=14 |issue=7 |pages=589–592 |doi=10.1016/0042-6989(74)90049-2 |pmid=4419807}}</ref><ref>Estimates of E<sub>2</sub> vary quite a bit. The approximate value of 2 degrees is taken from Strasburger et al. (2011), Table 4. It results from Anstis's (1974) Figure 1, with the foveal value assumed to be the standard 20/20 acuity.</ref> At 2 degrees eccentricity, for example, acuity is half the foveal value.

Visual acuity is a measure of how well small details are resolved in the very center of the visual field; it therefore does not indicate how larger patterns are recognized. Visual acuity alone thus cannot determine the overall quality of visual function.<ref>{{Cite journal |last1=Kandel |first1=Himal |last2=Nguyen |first2=Vuong |last3=Piermarocchi |first3=Stefano |last4=Ceklic |first4=Lala |last5=Teo |first5=Kelvin |last6=Arnalich-Montiel |first6=Francisco |last7=Miotto |first7=Stefania |last8=Daien |first8=Vincent |last9=Gillies |first9=Mark C. |last10=Watson |first10=Stephanie L. |author-link10=Stephanie Watson (ophthalmologist) |date=2022 |title=Quality of life impact of eye diseases: a Save Sight Registries study |journal=Clinical & Experimental Ophthalmology |language=en |volume=50 |issue=4 |pages=386–397 |doi=10.1111/ceo.14050 |issn=1442-6404 |pmc=9303885 |pmid=35080803}}</ref>