Apparent OPR as a function of the attenuation factor f_atten. Results from datasets S1 and S2–S4 are shown as grey and black dots, respectively. Results from datasets H1–H2 are shown by black squares. Only S1 data with an S/N on I^HB > 20 are considered. For S2–S4 and H1–H2, the error bars for f_atten represent the range of values obtained for f= 2.3 and f= 3. The arrows drawn for H1–H2 indicate a possible underestimation of fatten for these datasets. The curves show the polynomial regressions that fit the OPR^RT versus values shown in Fig. 10C: red and green represent OPR = 2.5 and 3, respectively.