Results of ∑EW vs. mag difference fitting with three lines and V filter for each cluster keeping fixed the slope β_V = 0.73 ± 0.04 Å mag⁻¹ from the extended sample of 13 clusters. Top panel: selected sample of seven clusters and the dashed line shows the fitted β_V = 0.71 ± 0.05 Å mag⁻¹ (see Table 4) for this sample to make it clear that the difference is negligible. Typical error bars are indicated in the top left corner. Equivalent figures for the other filters are presented in the appendix.