Maps of continuum intensity (a, b)) with marked penumbral boundaries (1U and 1P in a; 3U and 3P in b)). The white arrows point to the disc centre, and their length is proportional to the heliocentric angle. Plots c, d) show the magnetic field strengths dependent on the position angle (ψ) along the penumbral boundaries, where the  ∗  and  +  symbols represent the inner and outer penumbral boundaries, respectively. The colours of symbols correspond to the position on the boundary. The small black symbols are the fits of the measured magnetic field strengths with resulting formulas written in the plots. The magnetic field on the 1P and 3P boundaries is increased by 1 kG for display purposes.