Angle between the pupillary axis and the visual axis.
In practice, astigmatism that decreases spectacle best-corrected vision but is improved with the use of a gas-permeable lens and shows topographic corneal irregularity.
A spheroidal shape whereby the curvature is flattest centrally and gradually steepens toward the periphery. An oblate shape is similar to grapefruit, or a sphere flattened at the poles.
A spheroidal shape whereby the curvature is steepest centrally and gradually flattens toward the periphery. A prolate shape is similar to the pole of an egg.
Value of the difference in corneal asphericity between peripheral and central rays.
Higher-order aberration where paraxial rays focus in a different plane than peripheral rays.
Method to describe irregular astigmatism quantitatively, most frequently with Zernike polynomials.