Aspherical lenses - binoculars
Traditionally, the round shape of a lens is always a part of a (large) circle: a spherical lens. In the past, it was not possible to produce anything other than round lenses because of the limitations in production.
However, nowadays, it is possible to manufacture lenses that deviate from this spherical shape thanks to advanced grinding and polishing or pressing techniques. These modern lenses are called spherical lens. Aspherical lenses are able to correct optical aberrations more effectively. Furthermore, the same correction can be achieved at a lower cost.
Astigmatism is an optical aberration that can be effectively corrected with aspherical lenses.