In lens design, what phenomenon results from light entering at oblique angles?

In lens design, the phenomenon that results from light entering at oblique angles is known as marginal astigmatism. When light rays strike a lens at angles other than the perpendicular (or normal) to its surface, they do not converge at the same point, leading to different focal points for different meridians. This results in an image that may appear blurred or distorted, particularly towards the edges, because the lens does not focus all rays to a single spot. Marginal astigmatism is particularly significant in systems with wide apertures or in the peripheral regions of lenses, making it crucial to consider in high-quality optics design. Understanding this concept allows lens designers to optimize performance and correct for these aberrations in both photographic and optical applications.