There are four other key types of aberration: spherical and coma aberration, astigmatism, curvature of field, and distortion. Together with chromatic aberration, these phenomena make up what are known as Seidel's five aberrations. Spherical aberration refers to the blurring that occurs as a result of light passing through the periphery of the lens converging at a point closer to the lens than light passing through the center. Spherical aberration is unavoidable in a single spherical lens, and so aspherical lenses, whose curvature is slightly modified towards the periphery, were developed to reduce it. In the past, correcting spherical aberration required the combination of many different lens elements, and so the invention of aspherical lenses enabled a substantial reduction in the overall number of elements required for optical instruments.