Top panel: comparison between the spectroscopic and photometric photon indices for 30 quasars randomly extracted from the SDSS–4XMM sample and the quasars of the z ≃ 3 sample (Nardini et al. 2019, marked with open star symbols). Bottom panel: difference between the spectroscopic and photometric photon index values as a function of redshift. The mean ΔΓ_X = Γ_X, spectro − Γ_X, photo value and its 1σ dispersion are shown with the red and dashed lines, respectively. Points are colour-coded by the number of net counts. Although the Γ_X distribution along the one-to-one relation is rather scattered, the ΔΓ_X does not seem to show a clear trend with redshift.