Program FOCUS calculates the electric and magnetic fields in the vicinity of the astigmatic lens focus after the technique of Richards and Wolf. All input and output is via text on screen or files on disk. The incident light may have linear, circular, elliptical, radial, or azimuthal polarization and may illuminate the lens uniformly or with a Gaussian intensity profile. The algorithm is described in detail in the accompanying 27 page manual that describes the underlying physical equations upon which the program is based as well as providing many examples, often taken from the literature, demonstrating some of calculations that are possible.