In Against-the-rule astigmatism

ASTIGMATISM REGULAR ASTIGMATISM The astigmatism is regular when the refractive power changes uniformly from one meridian to another. Types of regular astigmatism Depending upon the axis and the angle between the two principal meridia, regular astigmatism can be classified into the following types: 1. With-the-rule astigmatism: In this type the two principal meridia are placed at right angles to one another but the veical meridian is more curved than the horizontal. Thus, correction of this astigmatism will require the concave cylinders at 180deg +- 20deg or convex cylindrical lens at 90deg +- 20deg. This is called &;with-the-rule&; astigmatism, because similar astigmatic condition exists normally (the veical meridian is normally rendered 0.25 D more convex than the horizontal meridian by the pressure of eyelids). 2. Against-the-rule astigmatism: refers to an astigmatic condition in which the horizontal meridian is more curved than the veical meridian. Therefore, correction of this astigmatism will require the presciption of convex cylindrical lens at 180deg +- 20deg or concave cylindrical lens at 90deg +- 20deg axis. Reference :- A K KHURANA; page:-36