Impact of the line shape (upper panels) and the presence of multiple lines (lower panels) on the predicted Δmag − λ₀ curves. The left panels show different input spectra, and the right panels the corresponding Δmag – λ₀ curves. “g”, “l”, and “b” in the legend refer to Gaussian, boxcar, and Lorentzian, respectively, with the succeeding numbers stating the line FWHM in kms^-1. The presence of multiple lines is demonstrated for the relevant example of the [N ii] doublet bracketing Hα, with the values in the legend corresponding to the assumed [N ii]λ6583 to Hα ratios. All the shown results are based on the same filter combination as used for Fig. 1.