Response functions of Stokes I and V to v_LOS, B and T in wavelength and optical depth, τ. Each response function is normalized with respect to the maximum absolute value. The dashed horizontal line emphasizes the location of log τ_5000 Å = 0. These response functions were computed by SIR, using profiles produced in synthesis mode from a FALC model atmosphere with γ and ϕ fixed at 45°, B at 300 G and v_LOS at 0 km s⁻¹.