  The standard treatment of gravitational lensing by a point mass lens M is
based on a weak-field deflection angle a = 2/x, where x = (r c^2)/(2 G M) with
r the distance of closest approach to the mass of a lensed light ray. It was
shown that for a point mass lens, the total magnification and image centroid
shift of a point source remain unchanged by relativistic corrections of second
order in 1/x. This paper considers these issues analytically taking into
account the relativistic images, under three standard lensing configuration
assumptions, for a Schwarzschild black hole lens with background point and
extended sources having arbitrary surface brightness profiles. We apply our
results to the Galactic black hole for lensing scenarios where our assumptions
hold. We show that a single factor characterizes the full relativistic
correction to the weak-field image centroid and magnification. As the
lens-source distance increases, the relativistic correction factor strictly
decreases. In particular, we find that for point and extended sources about 10
pc behind the black hole (which is a distance significantly outside the tidal
disruption radius of a sun-like source), the relativistic correction factor is
miniscule, of order 10^{-14}. Therefore, for standard lensing configurations,
any detectable relativistic corrections to microlensing by the Galactic black
hole will most likely have to come from sources significantly closer to the
black hole.
