Average observed and theoretical Ca profiles with different filters. Top panel: average observed Ca profile at disc centre (black line). Purple/blue/orange/red dashed lines: Gaussians with FWHM of 1 nm, 0.3 nm, 0.22 nm, and 0.03 nm, respectively, centred at 396.85 nm. Purple/blue/orange/red solid lines: multiplication of the observed profile with the filter transmission. Bottom panel: multiplication of NLTE FALC/FALF profiles (blue-dash-triple-dotted/red-dash-dotted lines) with a 0.3 nm filter.