Derived optical depth of the foreground extinction, for different types of carbonaceous grains and size distributions. The thick red and blue curves utilize the Milky-Way size distributions from Weingartner & Draine (2001) for the R_V = 3.1 and R_V = 5.5 extinction laws, respectively, using PAH and graphite grains. The thick green curve utilizes the MRN size distribution with graphite grains. All three curves include the silicate contribution derived by Boley et al. (2013). For comparison, the synthetic extinction curves of Weingartner & Draine (2001), which differ in their silicate content, are shown as thin red (R_V = 3.1) and blue (R_V = 5.5) curves.