Different degrees of curvature of the corneal surface are the main cause of regular astigmatism, which is usually congenital. Some mild astigmatism is only about 0.25 diopters, which may be the result of upper lid pressure on the eye, and is physiological in scope and generally does not require clinical correction. Acquired astigmatism is more common and is mostly due to: 1) corneal disorders such as corneal clouding, cone cornea, keratitis and corneal ulcers; 2) certain surgeries such as cataract removal, corneal transplantation, strabismus correction, pterygium excision, etc.; 3) eyelid tumors pressing on the cornea, finger pressure on the eye, etc. In addition, different degrees of bending of the two main axes of the lens and changes in the refractive index of early onset senile cataracts are also causes of astigmatism.