Synthesis of three spectral regions: C₂ (λ5165, left panels) and CN (λ4215, middle panels) molecular band heads, and the O atomic line at λ6300.30 (right panels). Two dwarf stars with different values of spectral S/N are shown: HD 168603 (S/N = 156, T_eff = 5399 K, [Fe/H] = 0.11 dex, top panels) and HD 190771 (S/N = 299, T_eff = 5817 K, [Fe/H] = 0.14 dex, bottom panels). These are the S/N per pixel computed at λ5550. Six spectra computed for different abundances (the N abundance is changed in the case of the CN band) and separated by 0.1 dex are plotted. The differences between observed and computed spectra (O−C) are also plotted in the bottom of each panel.