  The current modelling of single microlensing light curves neglects the
possibility that only a fraction of the light is due to the lensed star, the
remaining being due to a close, unresolved blend, which may be related or
unrelated to the lens. Unfortunately, the effects of blending are significant
as all microlensing experiments choose very crowded fields as their targets. In
this paper we point out a strong degeneracy of the fitting procedure which
makes it practically impossible to detect the presence of a blend by purely
photometric means, except in a small part of the parameter space. Some blends
may be detected by astrometric means, but the majority have to be corrected for
statistically. The luminosity function reaching well below the ground based
detection limit (with the HST) would be very also important and this could be
determined with the repeating microlensing events. If no correction is made
then the event time scales, the lens masses, and the optical depth are all
systematically underestimated.
