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Standardised charts mean vision recording known as visual acuity can be more reliably verified.
However, effects such as pupil size, contrast, optotype used, refractive error can cause variations.
If an observer has 6/6 visual acuity (or 20/20 vision ) the 6/6 size letters can be resolved at 6 metres (or 20 feet being 6 metres)
The direction of the E discriminated at different sizes is one type of eye test chart used in eyesight tests. The minimum angle of resolution is one minute of arc is ‘normal’ is by convention.
At this distance, the letters representing “normal” or 6/6(metres) or 20/20(feet) acuity subtend an angle of five minutes of arc, and the thickness of the lines and of the spaces between the lines subtends one minute of arc.
eg 20/40 or 6/12 vision (being poorer vision than 6/6 or 20/20 ) means only the larger 6/12 letter can be resolved at the 6-metre testing distance. Someone with so-called 6/6 visual acuity could resolve the 6/12 letter from a distance of 12 metres whereas
20/10 vision or 6/3 vision (better vision) means the letter can be seen only at 3 metres by the 6/6 limited person versus 6 metres for this better standard.