Technical requirements: V, I images down to a limiting magnitude of ( ) in a square degrees field.
Relaxation time in open clusters are two order of magnitudes shorter than in globular clusters. This means that for most of the open clusters, we expect that in times of the order of 106 years, energy equipartition has brought to a redistribution of the stars within the cluster, with the most massive stars preferentially located in the inner regions. Interaction with the galactic potential, differential rotation, etc., make quite difficult to model this dynamical evolution. As in the case of globular clusters, but to a much larger extent, the mass segregation effect must be well mapped in any open cluster for which we want to know the initial mass function from the present day (local) mass function. It is clear that, the study of the open cluster mass function can be satisfactorily carried out ONLY with a wide field imager for the determination of the field star contamination and the mass segregation effects.