With the astigmatism the cornea instead of having a shape of spherical type, in some areas can have different curvatures.
In practice, if we imagine the cornea crossed by many axes intersecting at the center, we could find to have that not all axes have the same curvature.
As a result of this we will have the whole image that is being formed on the retina will be normal but the portion of light which passes through the axis with different curvature arrive at distorted retina or forward or backward to the retina.
Foto concessione: Dott. C. Benvenuti
The subject, therefore, perceives an image more or less misty.
Astigmatism is corrected with appropriate lenses such as the cylindrical, which are positioned on the frame so as to correct only the axis with different curvature.