<prompt> What is regular astigmatism?Astigmatism in which the principal meridians are parallelAstigmatism in which the principal meridians are perpendicularAsymptomatic astigmatismAstigmatism as a result of cataract surgery  <response>: Ans. is bi.e., Astigmatism in which the principal meridians are perpendicular Types of AstigmatismA Based on axis of the principal meridians1 Regular stigmatism : Principal meridians are perpendicularo With-the-rule astigmatism-the vertical meridian is steepest.o Against-the-rule astigmatism-the horizontal meridian is steepest.o Oblique astigmatism-the steepest curve lies in between 120 and 150 degrees and 30 and 60 degrees.2 Irregular astigmatism - principal meridians are not perpendicular.B Based on focus of the principal meridians1 Simple astigmatismo Simple hyperopic astigmatism - first focal line is on retina while the second is located behind the retinao Simple myopic astigmatism - first focal line is in front of the retina while the second is on the retina.2 Compound astigmatismo Compound hyperopic astigmatism-both focal lines are located behind the retinao Compound myopic astigmatism-both focal lines are located in front of the retina.3 Mixed astigmatism - focal lines are on both sides of the retina straddling the retina. 