Visual acuity - Wikipedia
https://en.wikipedia.org/wiki/Visual_acuity
OverviewExpressionDefinitionMeasurementPhysiologyMeasurement considerations"Normal" visual acuityOther measuresVisual acuity is often measured according to the size of letters viewed on a Snellen chart or the size of other symbols, such as Landolt Cs or the E Chart. In some countries, acuity is expressed as a vulgar 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…
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