  Source blending in microlensing experiments is known to modify the Einstein
time of the observed events. In this paper, we have conducted Monte-Carlo
calculations, using the analytical relationships derived by Han (1999) to
quantify the effect of blending on the observed event time distribution and
optical depth. We show that short-time events are affected significantly by
source blending and that, for moderately blended sources, the optical depth
$\tau$ is globally overestimated, because of an underestimation of the
exposure. For high blending situations, on the opposite, blending leads to an
{\it under}estimation of the optical depth. Our results are in agreement with
the most recent optical depth determinations toward the Galactic Center of the
MACHO collaboration (Popowski et al. 2004) and the OGLE-II collaboration (Sumi
et al. 2005) that use clump giants (less affected by the blending effect) as
sources. The blending-corrected, lower optical depth toward the Galactic Bulge
is now in good agreement with the value inferred from galactic models,
reconciling theoretical and observational determinations.
