Equivalent width contribution functions for four distinct iron lines for the 3D (solid line) and counterpart 1D (dashed line) synthesis. Integrals of the curves yield the equivalent width of the Fe i line. Top left: a high excitation, weak Fe i line. Top right: a high excitation, strong Fe i line. Bottom left: a low excitation, weak Fe i line. Bottom right: a low excitation, strong Fe i line. These lines were chosen from our sample to illustrate the effect that excitation potential has on formation depths of Fe i lines. Both the 3D and 1D synthesis were given the same input transition probability (log gf) and excitation potential in each case. While the high excitation lines show good agreement in line strength, the low excitation lines do not. This helps to explain the behaviour shown in Fig. 5.