Upper panels: abundances obtained for individual Ca lines. The black error bars show the star-to-star median of the 1D–LTE analysis of the APOGEE data. The grey open circles and grey dots are literature data from respectively Roederer et al. (2014) and Bensby et al. (2014) from optical dwarf spectra. Bottom panels: corrections obtained with the 1D–NLTE models from this work (red squares) with the 3D–LTE models of Ludwig et al. (in prep.; green circles).