On the usual approximation used in the Rayleigh-Sommerfeld diffraction theory.

The complete Rayleigh-Sommerfeld scalar diffraction formula contains (1 - ikR) in the integrand. Usually the wavelength is small compared with the distance of the observation point from the aperture and (1 - ikR) is approximated by -ikR alone. Other approximations usually made in the Rayleigh-Sommerfeld formula are addressed as well. Closed-form solutions, without approximations, are possible wherein interesting consequences of these approximations become apparent.