Question

What are cylindrical lenses that are used during the visual defect astigmatism and their function?

Answer 1

Those who suffer from astigmatism cannot see objects clearly, when the light from certain parts falls on the retina and the rest falls on either behind the retina or in front of it.
This is caused by the irregular curvature of the cornea.

Since the problem is that the cornea or the eye lens are not symmetrical, the solution is to use eye glasses whose lenses are not symmetrical in a complementary way. An extreme version of this is a cylinder lens – a shape that is circular in one direction where it acts as a normal lens and flat in the other direction where it has no focusing properties. The strength and direction of the cylinder are designed to compensate for the asymmetry of the cornea or the eye lens. The curved portion of the cylinder lens may be positive or negative, depending on the detailed asymmetry in the eye.

It is easy to recognize eye glasses designed to deal with astigmatism, since acting by themselves they produce distorted images of any object. This is because they have different focal lengths (and therefore different magnifications) in different directions. Although people with mild hyperopia or myopia can often function without wearing glasses, people with astigmatism usually must wear glasses all the time. Otherwise they often get eye strain headaches caused by the eye muscles trying (and failing) to focus on the vertical and horizontal portions of an object at the same time.