Disc centre observations with different filter widths. Top row, left to right: Ca ii K Lyot image (0.03 nm), Hinode/FG broad-band Ca ii H image (0.3 nm), MFBD-reconstructed Ca image (1 nm), co-spatial MOMFBD line-core image of Fe i at 630.25 nm and the unsigned wavelength-integrated Stokes V signal of this line recorded with the GFPI. Bottom row, left to right: synthetic images derived by applying the corresponding filters of the top row to Ca spectra observed at disc centre. The synthetic image for the 1-nm-wide filter was obtained from the Ca spectra taken with a PCO camera inside of POLIS at a different time. All images are scaled individually inside their full dynamical range.