Top panel: observed spectra in grey, while the coloured curves are synthetic spectra with increasing iron abundance as the two central lines get deeper. The iron abundance varies by a total of 0.4 dex. The vertical lines show all the places where there are iron lines in the line list. Bottom panel: the two curves show the difference between the first synthetic spectrum and the second (Δ₂₁) and between the first synthetic spectrum and the third (Δ₃₁); this highlights where the change in iron abundance has an impact.