Aberrations of Optical Systems

defects in images occurring in optical systems. Aberrations are shown by the fact that optical images in some cases are blurred, do not match the object, or become colored. The most significant forms of aberrations are the following. Spherical aberration is a defect of the optical image in which the light rays near the optical axis and the rays traversing optical systems at some distance off the axis (for example, lenses) fail to converge at a single point. This aberration can be eliminated almost entirely in practice through the use of specially designed combinations of lenses. Coma is a defect of the optical image (the image of a point appears as an oblong asymmetric spot) that occurs when light rays traverse the optical system obliquely. In a simple lens, the dimensions of a spot are proportional to the square of the lens radius and to the angle of inclination of the light beam to the axis. At large angles of inclination of the beam to the axis, aberration becomes significant and is known as astigmatism. If a spherical light wave becomes deformed while traversing an optical system and ceases to be spherical, the bundle of rays becomes complex; the rays then intersect not at a single point but on two mutually perpendicular straight-line segments at some distance from one another. This is known as an astigmatic bundle and the phenomenon itself as astigmatism. The aberration of an optical system known as distortion is due to unequal linear magnification within the confines of the entire image field and breaks down the geometric similarities between the object and its image. Optical systems can have different types of aberrations at the same time. Correction of aberrations in complex optical systems, a different problem, is achieved through a proper combination of lenses. How specific forms of aberration are eliminated usually depends on the function and purpose of the optical system. The system aberrations enumerated here are known as geometrical aberrations. The wave nature of light is also responsible for some image defects in optical systems. These take place because of diffraction of light on diaphragms and irises, on lens mounts, and so forth. The effect of diffraction is usually much less than that of other system aberrations. There is also chromatic aberration caused by the dependence of the refraction index on the wavelength of light, which results in coloration of images in nonmonochromatic light.