Close-up of the 3900–4500 Å spectral range of the A3 V synthetic spectrum with a median of synthetic automatically normalised spectra (top panel) and residuals of normalisation errors (bottom panel). For this particular part of the spectrum, the average normalisation is significantly biased. These differences arise due to wide hydrogen absorption lines and strong metal lines which heavily blend in this spectral range. See Fig. 6 for the remaining results.