Forward simulation of FeH and Na i lines assuming two different distributions of surface magnetic fields. Thick black symbols: simulated observed spectrum; dashed violet line: computation with zero magnetic field; red line: computation with multi-component magnetic field shown on the bottom plots; blue line : computations with homogeneous magnetic field (i.e. f = 1) of the same average intensity as multi-component magnetic field. Columns 1–4 correspond to the assumed S/N of 100 and 500, respectively. Atmospheric parameters are the same for all simulations. Open blue diamonds in the bottom plots show the true distribution of filling factors used to simulate observations.