(Fe5335,Hβ) diagnostic plane. Left panel: (Fe5335,Hβ) diagnostics colour-coded according to metallicity for the templates library, considering galaxies with r-band light-weighted age > 2 Gyr. Right panel: (Fe5335,Hβ) diagnostics colour-coded according to the rms in metallicity. The light blue and white error bars represent the typical median 1σ error in measuring Fe5335 and Hβ at S/N_I,obs = 10 and S/N_I,obs = 30, respectively. The green arrow represents the effect on the indices of a 0.1% fraction of a stellar population with 70 Myr superimposed on a population with 7 Gyr, each with solar metallicity.