Mg_b-Fe24 index–index plot used to infer the [Mg/Fe] for the OptExt (black, left) and R50 (red, right) spectra. The F₂₄ index is the mean of the 24 iron lines present in the spectral range [3500−6500] Å. Coloured crosses are MILES SSPs with different ages and metallicities and colour-coded according to their [α/Fe] value. Black arrows show the direction of the variation expected by varying the stellar population parameters (age, Z, [α/Fe]). Both galaxies and SSP line strengths are calculated after convolving the spectra to a common resolution of σ = 300 km s⁻¹.