Astigmatism is an optical defect that causes blurred vision at all distances. It occurs when the cornea or lens of the eye has mismatched curves, instead of having one curve like a round ball, the surface is egg-shaped. This causes light rays to be refracted differently in different directions, resulting in two different images that overlap or combine and result in blurred vision. Astigmatism can be corrected by using glasses with a cylindrical lens.

Astigmatism can be classified into two types: regular and irregular astigmatism. In regular astigmatism, the principal meridians are perpendicular to each other (90-degree separation), while in irregular astigmatism, the principal meridians are not perpendicular to each other. Astigmatism can also be classified based on its cause, such as corneal astigmatism, which is caused by an irregularly curvy shape of the front surface of the eye, and lenticular astigmatism, which is caused by a lens with mismatched curves.

Symptoms of astigmatism may include blurred vision, eye strain, headaches, and difficulty seeing at night. Astigmatism is a common vision problem that may occur in combination with nearsightedness or farsightedness. It is often present at birth and may not require corrective action if it is not pronounced enough. However, when it is pronounced, treatment options include corrective lenses or surgery.

In summary, astigmatism is an optical defect that causes blurred vision at all distances due to the mismatched curves of the cornea or lens of the eye. It can be corrected by using glasses with a cylindrical lens, and it may occur in combination with nearsightedness or farsightedness. Treatment options include corrective lenses or surgery.