Scale errors in the photometry of some CCD standard fields

There is a scale error, i.e., a systematic dm which is a function of magnitude, in the CCD photometry of NGC 7790, NGC 4147 and NGC 7006 published by Odewahn et al. The scale error also exists in the CCD photometry of NGC 7790 by Petrov et al. The reason why this kind of error, which may only exist in photographic photometry, appears in a linear detector such as a CCD, is not immediately clear. If the CCD systems used by Odewahn et al. and Petrov et al. are really linear, maybe it is related to their use of the variable aperture technique in the former and the reduction method in the latter