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For small cosmological sources like supernovae, the matter distribution along the line of sight cannot accurately be described as a smooth fluid. This questions the applicability of standard cosmological lensing to that type of sources. In this talk, I will present a new method to take into account the granularity of the lensing matter. This method relies on a description of small-scale lensing as a diffusion process; the Sachs and Jacobi equations are recast in the form of Langevin equations for which Fokker-Planck-Kolmogorov equations will be derived. I will then use this formalism to derive some analytical properties of the average and dispersion of the angular diameter distance. Finally, applying the formalism to a particular case of random Swiss-Cheese models, I will present the results of a post Kantowski-Dyer-Roeder approximation, and discuss the shortfalls and future outlooks for the method.