  The images provided by the Advanced Camera for Surveys at the Hubble Space
Telescope (ACS/HST) have the amazing spacial resolution of 0".05/pixel.
Therefore, it is possible to resolve individual stars in nearby galaxies and,
in particular, young blue stars in associations and open clusters of the recent
starburst. These data are useful for studies of the extragalactic young
population using color magnitude diagrams (CMD) of the stellar groups. However,
even with the excellent indicated spatial resolution, the blending of several
stars in crowded fields can change the shape of the CMDs. Some of the blendings
could be handled in the cases they produce particular features on the stellar
PSF profile (e.g. abnormal sharpness, roundness, etc). But in some cases, the
blend could be difficult to detect, this is the case, were a pair or several
stars are in the same line of sight (e.g. observed in the same pixel). In this
work, we investigated the importance of the blending effect in several crowded
regions, using both numerical simulations and real ACS/HST data. In particular,
we evaluated the influence of this effect over the CMDs, luminosity functions
(LFs) and reddening estimations obtained from the observations.
