[Fe/H] derived from lines in the spectrum of HD 183658 as a function of excitation potential (upper panel) and normalized equivalent width: EW_norm. = EW·6000.0 /λ [Å] (lower panel). Open circles refer to Fe i lines and filled (red) circles to Fe ii lines. The full drawn lines show linear fits to the Fe i data for the derived parameters of the star. The dashed lines show changes in the slopes if T_eff is decreased by 30 K (upper panel) and the microturbulence is decreased by 0.1 km s^-1 (lower panel).