<P> Acuity = 1 / gap size (arc min) </P> <P> 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 . Emmetropic 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 astigmatism, ocular injuries, or presbyopia . </P> <P> Visual acuity is measured by a psychophysical procedure and as such relates the physical characteristics of a stimulus to a subject's percept and his / her resulting responses . Measurement can be by using an eye chart invented by Ferdinand Monoyer, by optical instruments, or by computerized tests like the FrACT . </P> <P> Care must be taken that viewing conditions correspond to the standard, such as correct illumination of the room and the eye chart, correct viewing distance, enough time for responding, error allowance, and so forth . In European countries, these conditions are standardized by the European norm (EN ISO 8596, previously DIN 58220). </P>

What is m unit notation in low vision