Range of R_V,eff produced by clumpy ISM models. Our models are indicated by filled blue circles. These are compared with the observed values of R_V reported in Fitzpatrick & Massa (2007; grey full circles) taking τ_V,eff = A_V/ 1.086. The thick black dashed line indicates the R_V of the dust cross-sections. Except for a small fraction of outliers, which increases toward large optical depth, clumpiness has little effect on typical interstellar extinction curves.