Absolute magnitude M_r vs. mean effective surface brightness ⟨μ⟩_eff,r in r-band. Symbols are the same as in Fig. 5. The black error bars in the left panel illustrate the statistical errors, while the arrows indicate the systematic error, as derived from analysing artificial BCD images (see the Appendix). The error bars are placed on the abscissa so that they correspond to the magnitude of the respective set of artificial BCDs. The arrows point in the direction towards which the measurements would have to be shifted to correct for the systematic error. Note that the left arrow is almost invisible due to its very low value.