Higher order corrections to lensing parameters for extended gravitational lenses

We discuss the contribution to the characteristic lensing quantities, i.e. the deflection angle and Einstein radius, due to the higher order terms (e.g. the gravitomagnetic terms) considered in the lens potential. The cases we analyze are the singular isothermal sphere and the disk of spiral galaxies. It is possible to see that the perturbative effects could be of the order 10^{-3} with respect to the ordinary terms of weak field and thin lens approximations, so that it is not a far hypothesis to obtain evidences of them in a next future by suitable experiments.