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Published online by Cambridge University Press: 25 May 2016
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.