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Base data: Physiology.
The outside diameter of the ring seen from a distance of 5m is 5 arc minutes, and the ring is designed so that if you can distinguish the ring’s size and breaks so that they are 1 arc minute, then your visual acuity is 1.0.
Reciprocal of distinguished visual angle is visual acuity
Seen in eye-sight tests, the “Landolt Ring” is a ring that looks like a tumbling letter C. When viewing a ring equivalent to visual acuity of 1.0 from a distance of 5m, the visual angle of the break is 1 arc-minute. The reciprocal of visual angle is visual acuity. Therefore, if the visual angle is 1 arc-minute, then visual acuity is 1.0, and if the visual angle is 2 arc-minutes, visual acuity is 1/2 or 0.5.
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