  We have completely mapped the globular cluster NGC 1851 with VI CCD
photometry, obtaining a CMD for ca. 20500 stars. From the apparent luminosity
of the HB we derive a distance modulus (m-M)o=15.44+/-0.20. The presence of 7
"supra-HB" stars is confirmed. Six of them are identified as evolved
descendants from HB progenitors. Synthetic HR diagrams demonstrate that the
problem of the HB morphology could be solved by assuming a bimodal efficiency
of the mass loss along the RGB. We find evidence that the radial distribution
of the blue HB stars is different from that of the red HB and SGB stars. The
first measurement of the mean absolute I magnitude for 22 known RR Lyr
variables (<M(I)>=0.12+/-0.20 at [Fe/H]=-1.28), is given. We found 7 new RR Lyr
candidates (6 ab type and 1 c type), lowering the ratio N(c)/N(ab) to 0.38.
  From a sample of 25 globular clusters (including NGC 1851) a new calibration
for DV(bump,HB) as a function of metallicity is derived, and we find some
evidence for an age-metallicity relation among GCs. We identify 13 blue
straggler stars, which do not show any sign of variability. The blue stragglers
are less concentrated than the SGB stars with similar magnitudes for r>80".
Finally, a radial dependence of the LF, a sign of mass segregation, is found.
Transforming the LF into a mass function and correcting for mass segregation by
means of multi-mass King-Michie models, we find a global MF exponent
x(0)=0.2+/-0.3.
