Simulations of the PDF of an observed cloud with an originally perfectly lognormal core and a power-law tail (solid line). The representation by a finite-size map introduces some uncertainties (dotted line). The addition of an ΔA_v = 1.0 (3) contamination distorts the shape of the PDF and observational noise adds a low-exctinction component. The deviation point A_v (DP) shifts by ~4 mag for a contamination of ΔA_v = 1.0 (3). The fit of the resulting PDF by a lognormal core and a power-law tail does not recover the initial parameters.