X‐ray flux in the 0.5–2 keV band vs. R‐band magnitude for the CDFS‐sources (larger symbols) and the ultradeep ROSAT survey in the Lockman Hole (smaller symbols). Objects are marked according to their X‐ray classification for the CDFS sources and the original classification for the Lockman Hole sources: squares correspond to QSOs, circles to AGNs, and triangles to galaxies. Type 1 and type 2 AGNs/QSOs have open and filled symbols, respectively. Stars are marked with star symbols. Crosses refer to spectroscopically not securely identified CDFS counterpart candidates that we observed in our program, plus symbols mark objects we did not observe. The solid lines correspond to an X‐ray–to–optical flux ratio index (as discussed in § 3) of 1, 0, and −0.4.