Aberrations are errors in an image that occur because of imperfections in the optical system. Another way of saying this is that aberrations result when the optical system misdirects some of the object’s rays. Optical components can create errors in an image even if they are made of the best materials and have no defects. Some types of aberrations can occur when electromagnetic radiation of one wavelength is being imaged (monochromatic aberrations), and other types occur when electromagnetic radiation of two or more wavelengths is imaged (chromatic aberrations). The origins and consequences of chromatic radiation were discussed in the previous section.
Monochromatic aberrations can be grouped into several different categories: spherical, coma, astigmatism, field curvature, and distortion.The idea of reference sphere is often used in discussions of aberrations. For all spheres, a ray drawn perpendicular to the sphere’s surface will intersect the center of the sphere, no matter what spot on the surface is picked.
A reference sphere isn’t a physical structure; it’s just a mathematical construct that the wavefront of the electromagnetic radiation is compared to. If the electromagnetic wavefront has the shape of the reference sphere, then the wavefront will come to a perfect focus at the center of the sphere. Remember that the definition of a ray specifies that rays are drawn perpendicular to the wavefront. All of the rays associated with a spherical wavefront will intersect at the center of the sphere. If the wavefront is not spherical, some of the rays will pass through the center of the sphere.
By comparing the wavefront of the electromagnetic radiation with the reference sphere, it is possible to determine what aberrations are present in an image and how severe they are.
Spherical aberrations occur for lenses that have spherical surfaces. Rays passing through points on a lens farther away from an axis are refracted more than those closer to the axis. This results in a distribution of foci along the optical axis.