Effect of variable far-infrared opacity on I_λ/ ℛ for the C11 (left) and J13 (right) models. The same five SEDs as Fig. 1 are shown, except the median SED, using the same colour and line style scheme. The normalisation is the same as Fig. 4. The observations are plotted in grey behind the models; the standard dust models are the solid lines with filled polygonal symbols and the modified dust models are the dashed lines with empty symbols. Numbers show the multiplicative factor of far-infrared opacity.