ABSTRACT
This entry begins with a brief introduction to Gaussian imaging, and then introduces Petzval image that accounts for the slight focus error of an off-axis point object. The concept of ray and wave aberrations is explained and a relationship between the two is given. Expressions for the wave aberrations are given when the image is observed in a defocused image plane or when the wavefront is tilted. The aberrations of a rotationally symmetric imaging system are described in terms of a power series of Zernike polynomials, with emphasis on primary aberrations. How the expressions for the primary aberrations of a system change when its aperture stop is shifted are given, and analytical expressions for the aberrations of some simple systems are given as examples. How the aberrations of a system may be observed is also described briefly.
