Visual acuity (VA) is acuteness or clearness of vision, which is dependent on the sharpness of the retinal focus within the eye and the sensitivity of the interpretative faculty of the brain.
Visual acuity is a measure of the spatial resolution of the visual processing system and is usually tested in a manner to optimise and standardise the conditions. To this end, black symbols on a white background are used (for maximum contrast) and a sufficient distance allowed to approximate infinity in the way the lens attempts to focus. Twenty feet, or six metres, is essentially infinity from an optical perspective (the difference in optical power required to focus at 20 feet (6.1 m) versus infinity is only 0.164 diopters). Whilst in an eye exam, lenses of varying powers are used to precisely correct for refractive errors, using a pinhole will largely correct for refractive errors and allow VA to be tested in other circumstances. Letters are normally used (as in the classic Snellen chart) as most people will recognise them but other symbols (such as a letter E facing in different directions) can be used instead.
In the term "20/20 vision", the numerator refers to the distance in feet between the subject and the chart. The denominator indicates the size of the letters, specifically it denotes the separation at which the lines that make up those letters would be separated by a visual angle of 1 arc minute, which for the lowest line that is read by an eye with no refractive error (or the errors corrected) is usually 20 feet (6.1 m). The metric equivalent is 6/6 vision where the distance is 6 metres. This means that at 20 feet or 6 metres, a typical human eye, able to separate 1 arc minute, can resolve lines with a spacing of about 1.75mm. 20/20 or 6/6 vision can be considered nominal performance for human distance vision; 20/40 or 6/12 vision can be considered half that acuity for distance vision and 20/10 or 6/3 vision would be twice normal acuity. The 20/x number does not directly relate to the 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.
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