Effective optical depth of self-gravitating spheres and cylinders as function of the mean optical depth for several assumptions for the over pressure . The overpressure is varied from 10⁰ to 10³ in steps of . The dashed line shows the effective optical depth for clouds with over pressure , the over pressure for critical stable spherical clouds. The thin grey lines show the approximations for low and high mean optical depths. The effective extinction of the cylinders is also compared to the corresponding values of a homogeneous sphere of the same mean optical depth (dotted line).