Long time since I did any optics but this basically just convolution. The impact at the output plane of dust/imperfections on the lens is (approximately) a 2D fourier transform of the dust/imperfections. If you have a single dust particle (essentially an impulse) on the lens this will be convolved into a dc response at the focal plane.

[1] http://en.wikipedia.org/wiki/Fourier_optics#Fourier_transfor...