We investigate blending , binarity and photometric biases in crowded-field CCD imaging .
For this , we consider random blend losses , which correspond to the total number of stars left undetected in unresolved blends .
We present a simple formula to estimate blend losses , which can be converted to apparent magnitude biases using the luminosity function of the analyzed sample .
Because of the used assumptions , our results give lower limits of the total bias and we show that in some cases even these limits point toward significant limitations in measuring apparent brightnesses of “ standard candle ” stars , thus distances to nearby galaxies .
A special application is presented for the OGLE-II BVI maps of the Large Magellanic Cloud .
We find a previously neglected systematic bias up to 0 \aas@@fstack { m } 2–0 \aas@@fstack { m } 3 for faint stars ( V \sim 18 \aas@@fstack { m } 0 - 19 \aas@@fstack { m } 0 ) in the OGLE-II sample , which affects LMC distance measurements using RR Lyrae and red clump stars .
We also consider the effects of intrinsic stellar correlations , i.e .
binarity , via calculating two-point correlation functions for stellar fields around seven recently exploded classical novae .
In two cases , for V1494 Aql and V705 Cas , the reported close optical companions seem to be physically correlated with the cataclysmic systems .
Finally , we find significant blend frequencies up to 50–60 % in the samples of wide-field exoplanetary surveys , which suggests that blending calculations are highly advisable to be included into the regular reduction procedure .