  We investigate the dark halo lens mass function (MF) for a wide class of
spheroidal non singular isothermal models comparing observed and observable
microlensing quantities for MACHO observations towards LMC and taking into
account the detection efficiency. We evaluate the microlensing observable
quantities, i.e. observable optical depth, number of events and mean duration,
for models with homogenous power - law MF changing the upper and lower mass
limits and the flattening of the dark halo. By applying the simple technique of
the inverse problem method we are then able to get some interesting constraints
on the slope $\alpha$ of the MF and on the dark halo mass fraction f made out
by MACHOs consistently with previous results.
