CCD direct imaging of fields around quasars is used as a method for locating galaxy clusters and groups associated with quasars. Average galaxy counts in the sky obtained from control fields are used to correct for background galaxies in the quasar fields. This correction allows one to derive the luminosity function (LF) of the associated galaxies at the redshifts of the quasars. It is demonstrated that using the derived LF and average galaxy count data, self-consistent models of the evolution

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... f the LF and galaxy counts can be obtained. Current data are best fitted, with a large uncertainty, by a qobetween 0.0 and 0.5 and an evolution in M*of −0.9±0.5 mag. It is found that the average environment of radio-loud quasars at z∼0.6 is about three times richer in galaxies than that of quasars at z∼0.4.