Equivalent hydrogen column density N_H versus extinction in the V-band A_V. For 90% of sources the uncertainties are lower than those represented by the grey error bars in the lower right corner of the figure. Median values of errors are indicated by the black error bars. The solid black, orange, and blue lines are linear fits to the entire sample, IR-excess sources only, and Class III objects only, respectively. The shaded area and dotted orange or blue lines indicate the 1σ confidence band of the fits. The dashed grey line corresponds to the N_H/A_V ratio of the diffuse ISM in the Milky Way, N_H = 2 × 10²¹ cm^-2 mag^-1·A_V.