Top and middle panels: same as Fig. 3, but for the spectral line width (FWHM) as retrieved from a Gaussian fit to the Stokes I profiles of Fe i 5250.2 Å. Bottom panel: influence of different approximations made during the spectral synthesis of the 30 G MHD data on the histogram of the spectral line width. The spectra were fully degraded in all three plotted cases. The black line corresponds to the histogram obtained when the filter transmission profile was approximated by an 85 mÅ Gaussian instead of the measured spectral PSF. The blue line corresponds to a spectral synthesis of only the 5250.2 Å line, i.e., the 19 neighboring lines were not synthesized. For comparison, the red line in the top panel is plotted again and corresponds to the 20-line synthesis and spectral degradation with the measured spectral PSF.