Rayleigh-Sommerfeld Fraunhofer Diffraction
  Treatments of Fraunhofer diffraction normally approximate the inclination factor in the diffraction integral by 1, but this is not necessary. In this paper, the Rayleigh-Sommerfeld diffraction integral is used and a complete description of Fraunhofer diffraction is given, with the minimum possible number of approximations made. A focused wave is examined before, at, and after its focal point to elucidate the exact role of the Fourier transform in describing Fraunhofer diffraction and to derive the Guoy phase shift. 
Robert L. Lucke