Disk-center (μ = 1) equivalent width contribution functions, ℬ(log τ_c), of a weak (top) and strong (bottom) Fe ii line with excitation potential χ = 10 eV, at λ 850 nm. The contribution functions have been computed for a single snapshot of the 3D model (solid), the corresponding ⟨ 3D ⟩ average model (dashed), and the associated 1D LHD model (dotted) used in this work. They have been transformed from the monochromatic to the Rosseland optical depth scale. In all cases, the saturation factor exp { − τ_ℓ } is properly taken into account (see Eq. (B.1)).