Illustration of the sensitivity of different passbands to stellar metallicity. Plotted is the difference in magnitude between a star with solar T_eff, log g, and metallicity, and one with [Fe/H] = −2.5 dex (left panel) or one with [Fe/H] = −0.5 dex (right panel) for three different log g values as indicated in the legend. The T_eff values for the models are chosen to agree with the metallicity and log g of the isochrones. See Sect. 3 for further details on how Δmag was calculated, which has been normalised at 5550 Å (as in Bond 1999). The four broad Johnson–Cousins passbands (UBVI) have been indicated (thick grey lines), as well as the Earth’s atmospheric cutoff (vertical dashed grey line).