In order to measure visual acuity, a test person must determine the orientation of the gap in a so-called Landolt ring (a circular ring with a gap or opening as a standard optotype or vision mark for vision tests). The angle of vision can be calculated from the size of the gap in the ring and the viewing distance. The observation distance results in an angle of vision (å), the reciprocal of which is the measure for visual acuity (S = 1: å). A visual acuity (Visus) of 1 equates to recognition of the gap under an angle of vision å = 1'.
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