Normalized response function of Stokes I (left) and Stokes V (right) to the magnetic field B, multiplied by 1/Δτ_C[10⁻⁵ G⁻¹], for the pair of Fe I absorption lines at 6301.5 and 6302.5 Å in the atmosphere inferred by the SPINOR 2D inversions for the penumbral pixel marked with a blue plus symbol in Fig. 3a. Some physical parameters for such an atmosphere are presented in Table 2. The horizontal dashed lines indicate the selected nodes for the SPINOR 2D inversion: log(τ) = 0, −0.8 and −2.