Distribution of spatially unresolved objects in the optical band in the log(f_x/f_r) versus g − i diagram. Left: the identified sample. Blue: AGNs with log(L_X) ≥  44, green: 44 ≥ log (L_X) ≥ 42, magenta: QSO2s – filled squares = X-ray selected – filled triangles = optically selected – encircled = Compton Thick (see Sect. 8.1), red: stars, cyan: accreting binaries. Right: the entire SDSS photometric sample. In this case, the colour codes the range of r magnitude. black:  < 18, red: 18–20, green, 20–21, blue:  > 21. We only show SDSS entries with a probability of identification with an X-ray source higher than 90%, g and i magnitudes brighter than 22.2 and errors on g − i  <  0.2.