Illustration of the determination of magnetic field strength B and filling factor f for HD 129333 (2009.02 epoch). The greyscale corresponds to the χ² of the fit to the three magnetically sensitive Fe I lines, as quantified by the side bar. The best fitting parameters, B = 3.5 kG and f = 0.4, are indicated with the cross. Solid lines show 68.3, 95.5, and 99.7% confidence limits. The dashed line corresponds to the B ⋅ f = const curve.