Comparison of the relations between the face-on optical depth and the gas surface density obtained in this paper (solid lines, Eqs. (12) and (13)), with the relation from Guiderdoni & Rocca-Volmerange (1987) (dashed line), and with the relation from Güver & Özel (2009) (dotted line). The coloured lines show metallicity-dependent relations according to the oxygen abundance scale shown on the right. For clarity, we have plotted the Güver & Özel (2009) relation only for solar metallicity.