  Strong lensing provides popular techniques to investigate the mass
distribution of intermediate redshift galaxies, testing galaxy evolution and
formation scenarios. It especially probes the background cosmic expansion,
hence constraining cosmological parameters. The measurement of Einstein radii
and central velocity dispersions indeed allows to trace the ratio D_s/D_ls
between the distance D_s from the observer to the source and the distance D_ls
from the lens to the source. We present an improved method to explicitly
include the two - component structure in the galaxy lens modeling, in order to
analyze the role played by the redshift and the model dependence on a nuisance
parameter, F_E, which is usually marginalized in the cosmological applications.
We show how to deal with these problems and carry on a Fisher matrix analysis
to infer the accuracy on cosmological parameters achieved by this method.
