Upper panel: comparison between the more metal poor and metal rich synthetic spectra from our library, in the region around the CaT lines. In both cases, the black line shows the spectra including the lines of all elements, while the red lines shows only the three CaT lines. The lower panels show the difference between the real EW, as measured in the red spectra, and the measured EW, as measured in the black spectra, for each of the three CaT lines as a function of EW and for three values of S/N.