Editing Visual acuity (section)

== "Normal" visual acuity{{anchor|Normal_vision}} ==
Visual acuity depends upon how accurately light is focused on the retina, the integrity of the eye's neural elements, and the interpretative faculty of the brain.<ref>{{Cite book |last1=Carlson |first1=N |title=Clinical Procedures for Ocular Examination |last2=Kurtz |first2=D |last3=Heath |first3=D |last4=Hines |first4=C |date=1990 |publisher=Appleton & Lange |isbn=978-0-07-184920-3 |location=Norwalk, CT |name-list-style=vanc}}</ref> "Normal" visual acuity (in central, i.e. foveal vision) is frequently considered to be what was defined by [[Herman Snellen]] as the ability to recognize an [[optotype]] when it subtended 5 [[minute of arc|minutes of arc]], that is Snellen's chart 6/6-metre, 20/20 feet, 1.00 decimal or 0.0 logMAR. In young humans, the average visual acuity of a healthy, [[refraction error|emmetropic]] eye (or [[Refractive error|ametropic]] eye with correction) is approximately 6/5 to 6/4, so it is inaccurate to refer to 6/6 visual acuity as "perfect" vision. On the contrary, Tscherning writes, "We have found also that the best eyes have a visual acuity 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."<ref name="Physiologic Optics" />

6/6 is the visual acuity needed to discriminate roughly 1 cycle per 1.6 arc-minutes (2.15 cycles per miliradian) – 3.5 mm at 6 metres. This is because a 6/6 letter, E for example, has three black limbs with two white spaces between them which corresponds to slightly more than 2 cycles (where 1 cycle is defined as the transition from the beginning of a black bar, to a white bar and back to a black bar) over the entire height of the letter. The significance of the 6/6 standard can best be thought of as the lower limit of normal, or as a screening cutoff. When used as a screening test, subjects that reach this level need no further investigation, even though the average visual acuity with a healthy visual system is typically better (eg. 6/4.5, 20/15, or 2.9 cycles/mrad). 

Some people may have other visual problems, such as severe [[visual field]] defects, [[color blindness]], reduced [[contrast (vision)|contrast]], mild [[amblyopia]], cerebral visual impairments, inability to track fast-moving objects, or one of many other visual impairments and still have "normal" visual acuity. Thus, "normal" visual acuity does not imply normal vision. The reason visual acuity is very widely used is that it is easily measured, its reduction (after correction) often indicates some disturbance, and that it often corresponds with the normal daily activities a person can handle, and evaluates their impairment to do them (even though there is heavy debate over that relationship).