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Visual acuity (VA) is the measurement of how well the entire vision system can recognize what it sees. Formally, it is defined as the reciprocal of the smallest angle that the visual system can resolve. | Visual acuity (VA) is the measurement of how well the entire vision system can recognize what it sees. Formally, it is defined as the reciprocal of the smallest angle that the visual system can resolve. | ||
It will depend on the amount of defocus that [[accommodation]] cannot overcome | It will depend on the amount of defocus that [[accommodation]] cannot overcome because it is outside of the focus range or due to other defects such as [[astigmatism]]. Defocus causes [[blur]] and [[double vision]]. | ||
For distance, this is typically measured with a [[Snellen chart]] and expressed as the distance you need to be from the chart to recognize the characters over the distance that someone with normal vision can recognize the characters. Lens or mirrors may be used to change the effective test distance in a smaller room. | For distance, this is typically measured with a [[Snellen chart]] and expressed as the distance you need to be from the chart to recognize the characters over the distance that someone with normal vision can recognize the characters. Lens or mirrors may be used to change the effective test distance in a smaller room. | ||
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With text that we are familiar with, the brain may clear up that text more than our vision would actually allow. | With text that we are familiar with, the brain may clear up that text more than our vision would actually allow. | ||
20/20 is "good enough" for most purposes, but not "perfect" in any way. Rather, 20/20 vision is slightly defective. It is possible to have 20/20 vision with 0.25 to 0.5 D of defocus, and 20/20 does not imply clear or sharp vision. As [[normalized]] correction typically | 20/20 is "good enough" for most purposes, but not "perfect" in any way. Rather, 20/20 vision is slightly defective. It is possible to have 20/20 vision with 0.25 to 0.5 D of defocus, and 20/20 does not imply clear or sharp vision. As [[normalized]] correction typically imposes 0.25 D of extra myopic defocus on [[20/20 correction]], it can be considered to be 0.25 to 0.75 D of undercorrection. | ||
Tscherning writes in ''Physiologic Optics: Dioptrics of the Eye, Functions of the Retina, Ocular Movements and Binocular Vision'': | Tscherning writes in ''Physiologic Optics: Dioptrics of the Eye, Functions of the Retina, Ocular Movements and Binocular Vision'': | ||
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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.</blockquote> | 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.</blockquote> | ||
==Reasons differing visual acuity== | ==Reasons for differing visual acuity== | ||
[[ | Non-zero [[refractive state]], which causes [[blur]] and [[double vision]], is only one factor of visual acuity. Various medical conditions can cause physical blockage/dispersion of light in the eye, problems detecting light in the eye, or problems with the visual processing that turns a series of electrochemical signals into a picture in our mind's eye. See an [[optometrist]] if your vision can't be corrected with [[refraction]]. | ||
==20/x== | ==20/x== | ||