ABSTRACT
The Modified Preliminary Drill also treats simple astigmatism, compound myopia and high astigmatism, and compound anisometropia and high astigmatism. The assumption is that the patient’s prescription is within the myopic model. The contact lens’ flatness factor to treat “compound astigmatism” is equal to the absolute value of the “sphere” if the astigmatism is low. If the astigmatism is high, the flatness factor is equal to the absolute value of the “sphere” plus a compensating value—usually 0.25 D. (Refer to the chapter Specifications.)
