Astigmatism is a type of refractive error. Because our eyes are not a perfect sphere, they are more like an egg shape, with different meridians having different refractive powers. Thus, after imaging, the axial images may form in front of or behind the retina, rather than forming a focal point on the retina. Normally, astigmatism of some small degrees and with good axial alignment may not significantly affect vision. However, high degrees of astigmatism, especially irregular astigmatism, will definitely impact visual quality and require the prescription of special lenses to correct the astigmatism.