Luminosity – metallicity relation of SF dwarfs. The dashed line shows the linear fit based on dIs in the LV and Virgo cluster with very precise T_e oxygen abundances (Vaduvescu et al. 2007). Most targets in the four Abell clusters match this linear trend, but have larger scatter due to larger uncertainties in the Hubble distances and the abundance indirect method. The cross in the upper left shows typical uncertainties in the two parameters.