Astigmatism and curvature of image field in lens imaging

The experiments P5.1.3.1 and P5.1.3.2 deal with aberrations of image sharpness. In a ray path parallel to the optical axis, paraxial rays are united at a different distance from abaxial rays. This effect, known as “spherical aberration”, is particularly apparent in lenses with sharp curvatures. Astigmatism and curvature of field may be observed when imaging long objects with narrow light beams. The focal plane is in reality a curved surface, so that the image on the observation screen becomes increasingly fuzzy toward the edges when the middle is sharply focused. Astigmatism is the phenomenon whereby a tightly restricted light beam does not produce a point-type image, but rather two lines which are perpendicular to each other with a finite spacing with respect to the axis.