Illustration of an observed Mg line (upper panels) and an Fe line (lower panels) and their respective 1D–LTE (red) and 1D–NLTE (blue) models in the solar spectrum (Livingston & Wallace 1991). Left panels: high-resolution (R > 100 000) andvery high signal-to-noise ratio (S∕N > 800) observations, while right panels: same spectrum at lower resolution (R ≈ 22 000) and lower signal-to-noise ratio (S∕N = 100). Although the high-resolution spectra clearly highlight some modelling issues around the core of the lines, the effect is less clearly visible in the lower resolution spectrum.