Difference between [Fe/H] derived from spectra of varying S/N (per pixel) for stars in NGC 6528. [Fe/H]_x indicates our value measured for each spectrum, while [Fe/H]_r indicate the value obtained in Sect. 5.1 for each star. The full line indicated the average difference evaluated as a running average. The shaded region indicates the 1σ difference, while the dashed-dotted line indicates the 2σ difference. Details on individual stars are given in Appendix E. Note that in this figure we do not distinguish between stars for which different methods to derive T_eff have been used (compare Fig.7).