Abstract. In a series of papers we have analyzed different aspects of the problem with systematic errors in stellar photometry as a function of spatial distribution of stellar images over the detector. In the present article we state the problem and show the general form of the spatial dependence of instrumental magnitudes. The influence of some optical elements on this phenomenon is discussed. Comments on possible reasons for this kind of systematic errors are presented also. Data are based on direct observations taken with a CCD in the RC focus of the 2-m telescope of the Bulgarian National Astronomical Observatory 'Rozhen'.

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