Efficient gravitational lens optical scalars calculation of black holes
  with angular momentum
  We provide new very simple and compact expressions for the efficient
calculation of gravitational lens optical scalars for Kerr spacetime which are
exact along any null geodesic. These new results are obtained recurring to well
known results on geodesic motion that exploit obvious and hidden symmetries of
Kerr spacetime and contrast with the rather long and cumbersome expressions
previously reported in the literature, providing a helpful improvement for the
sake of an efficient integration of the geodesic deviation equation on Kerr
geometry. We also introduce a prescription for the observer frame which
captures a new notion of \emph{center of the black hole} which can be used for
any position of the observer, including those near the black hole. We compare
the efficient calculation of weak lens optical scalars with the exact
equations; finding an excellent agreement.
