Astigmatism is defined as the uneven curvature of the cornea, the transparent outer part of the eye. We can describe it as a small bulge on the surface of an otherwise smooth rubber ball. Blurred vision may also be caused by a deformed cornea, which should be evenly curved.
Astigmatism is a congenital disorder where the cornea is unevenly curved, causing blurred vision. This unevenness causes the light rays to be dispersed on the retina in the focal point creating a blurred and unclear image. Astigmatism may have positive and negative values. It frequently accompanies myopia as well as hypermetropia. It is rectified using cylindrical dioptres.
This refraction defect is always rectified when treating spherical dioptric defects: hypermetropia and myopia.