Comparison of the behaviour of model Mgb, CaHK, Ca4455, [MgFe], Fe5015, and NaD absorption-line index strengths as a function of the model Hβ index strength. The shaded area represents XSL Padova00 models with varying spectral resolution (σ) from σ = 13 km s⁻¹ (the native XSL resolution) to σ = 60 km s⁻¹ (the minimum E-MILES resolution). Black lines represent E-MILES Padova00 model predictions, with dotted, dashed, and solid lines representing [Fe/H] = +0.2, 0.0, and −0.4 dex, respectively, measured at the original E-MILES resolution. The solid blue line represents predictions from the C18 solar-metallicity models.