Histograms of the unsigned radial magnetic flux density |B_r| in magnetograms reconstructed from Ca II K observations taken on 11 August 2001 (top), 16 July 2009 (middle), and 10 May 2012 (bottom). The observations used are from RP (black), MD1 (blue/purple for reconstructions with/without the normalisation of the contrast values to RP, see Sect. 3), SF (dark/light green for reconstructions with/without the normalisation), and SDO/HMI (orange, shown only in the bottom panel as these data are not available prior to 2010) archives. The histograms are produced for bins of 1 G. The vertical dashed lines mark the adopted B_sat value for each archive when using PMOD TSI as the reference, while the grey shaded surface denotes the 3-σ noise level (∼20 G) of the SDO/HMI magnetograms.