Difference in synthetic magnitudes from HR and XP spectra for SPSS, PVL, and selected NGSL using passbands of increasing FWHM (from top to bottom) to measure the flux around Hβ (left panels) and Hα (right panels), as a function of G_BP-G_RP colour. The passbands FWHM adopted in the various panels are, from top to bottom, 5, 8, 13, 18, 23 nm (Hβ), and 3, 6, 9, 12, 15 nm (Hα). The corresponding Rf values are reported in each panel.