Astigmatism has many causes, the main one is caused by changes in the curvature of the cornea of the eye. The main function of the cornea is to pass light and focus it so that it enters the pupil and reaches the retina at the bottom of the eye, where it becomes a focal point and is then reflected to the brain, enabling people to see the outside world clearly. Therefore, when the curvature of the hemispherical cornea changes, the external light does not form a focal point on the retina, but a post-focal line, which results in astigmatism. Astigmatism includes regular astigmatism, which is caused when the curvature of the cornea does not coincide with its vertical direction in a certain direction. There are many types of astigmatism, including simple hyperopic astigmatism, simple myopic astigmatism, compound hyperopic astigmatism, compound myopic astigmatism, and mixed astigmatism, etc. Most of them are caused by the different refractive power of the cornea, and they can be corrected by glasses because they coexist with myopia or hyperopia. There are two causes of irregular astigmatism due to uneven corneas and incorrect curvature: one is congenital and the other is due to acquired corneal disease. Such as. If an ulcer or trauma occurs in the cornea, and the scar is produced after healing, it will cause the cornea to be uneven and the curvature to be incorrect.