Comparison between input stellar metallicity and the EW predicted by Starburst99 and BPASS synthetic spectra (violet diamonds and filled white circles, respectively), for nine absorption line indexes typically adopted in the literature to calibrate the metallicity. We considered a continuous SFH for an age varying between 50 Myr and 2 Gyr (shaded region). The markers in each panel refer instead to an age of 100 Myr, which is taken for the final calibration of the 1501 and 1719 indexes (see Fig. ).