Column density at the center of a self-gravitating sphere or a cylinder as a function of the overpressure , where the mass is homogeneously distributed within the cloud radius. The column density and the extinction are given for an external pressure p_ext = 2  ×  10⁴   K/cm³. The asymptotic behaviors are shown as dashed lines. The symbols mark the positions of the accurate radiative transfer calculations. For comparison also the true column densities at the cloud centers is shown (light grey curves).