So far we have seen lenses that compensate myopia and hyperopia . These lenses are spherical are built from 2 radii (one for the first side and one for the second side of the lens), thus the ranking of the lens is the same if measured at 0 º and 90 º.
However, lenses that compensate for astigmatism are toric lenses. We saw that astigmatism occurs because the cornea is not completely spherical, but rather "seems" to a rugby ball. Therefore, the lens compensates for this defect is a ring or cylindrical lens. The graduation of this lens is different in its meridian perpendicular, ie, measured graduation at 0 º and 90 º will not match. Because of this, the value of astigmatism is always associated with an axis between 0 and 180 degrees as when riding a toric lens should be exactly that direction to form a sharp image.
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