The authors present VI CCD photometry for nine fields towards the centre of NGC 1851 covering an area of 22×22 arcmin2, out to 1.2 tidal radii. These were observed during two nights in February 1993 at the ESO 3.5-metre NTT telescope + EMMI. The sample consists of ≡20500 stars reaching V = 22 or I = 21.5 at the 50% completeness level. These data have been collected within a project of mapping the stellar distribution of a sample of galactic GCs, in order to detect the effects of dynamical evolution and to reconstruct the stellar initial mass function (IMF). The mass function (MF) slope of NGC 1851 is x = 1.0±0.5, in agreement with the general scenario of an MF depending on the position in the galactic gravitational potential and metal content of the cluster. The authors also plan to check the influence of environmental parameters such as concentration on the evolution of their stellar populations. As a by-product of this research they exploit their database to investigate the evolved stages of stellar evolution of low-mass stars.