  The evolution of the N=50 gap is analyzed as a function of the occupation of
the proton f5/2 and p3/2 orbits. It is based on experimental atomic masses,
using three different methods of one or two-neutron separation energies of
ground or isomeric states. We show that the effect of correlations, which is
maximized at Z=32 could be misleading with respect to the determination of the
size of the shell gap, especially when using the method with two-neutron
separation energies. From the methods that are the least perturbed by
correlations, we estimate the N=50 spherical shell gap in 78Ni. Whether 78Ni
would be a rigid spherical or deformed nucleus is discussed in comparison with
other nuclei in which similar nucleon-nucleon forces are at play.
