Which aberrations are categorized as "narrow beam" aberrations?

Marginal astigmatism and curvature of field are indeed categorized as "narrow beam" aberrations. This classification relates to how these aberrations affect light rays that converge at different points, causing image blurring when these rays pass through the optical system.

Marginal astigmatism occurs when rays entering the lens at different angles are focused at different points, leading to a loss of focus in certain parts of the image. This is particularly relevant in narrow beams of light, where control over the path and convergence of rays is essential to maintaining sharpness across the image field.

Curvature of field refers to the scenario where an image is not focused on a flat plane but rather on a curved surface. This aberration can cause straight lines to appear curved at the edges of the image, again affecting narrow beams differently than broader beams, as narrow beams exaggerate this effect due to their more confined light path.

Understanding these narrow beam aberrations is crucial, especially in applications requiring high precision, such as in optical instruments and imaging technologies, where clarity and accuracy are paramount.