Which aberration is produced when a narrow beam of light passes obliquely through a lens?

The phenomenon that occurs when a narrow beam of light passes obliquely through a lens is known as marginal astigmatism. This type of aberration arises when light rays that are incident on the lens at different distances from the optical axis focus at different points, leading to blur or distortion in the image.

In the case of marginal astigmatism, the oblique rays do not converge to the same focal point as rays that pass closer to the axis of the lens. This results in the formation of separate focus points for the different orientations of light rays—namely the tangential and sagittal foci—which can affect the clarity and quality of the image produced by the lens.

This aberration is particularly significant in optical systems where precision is important, such as in cameras or microscopes, because it can compromise the overall resolution of the image captured. Understanding marginal astigmatism is crucial for designing lenses that minimize such distortions and improve image quality.