What is Astigmatism and How Does It Occur? Astigmatism is a refractive error that causes blurred vision due to focusing problems in the eye. Irregular curvatures of the cornea or lens in the eye cause light not to focus properly. Astigmatism can be accompanied by other vision problems, such as myopia (nearsightedness) or hyperopia (farsightedness). Astigmatism can be combined with such refractive errors [...].