Foreground Galaxies and the Variability of Luminous Quasars

In order to look for an amplification bias (AB) by gravitational lensing caused by medium redshift (0.2 ≲ z ≲ 0.8) clusters or groups of galaxies, we compare galaxy counts in deep CCD images of highly luminous, high redshift QSOs with those in nearby control fields at a distance of 1 deg at the same galactic latitude. The total sample contains 37 objects up to now, from which one field had to be excluded because of a seeing difference between the QSO and control fields.