  Gravitational lensing is most often used as a tool to investigate the
distribution of (dark) matter in the universe, but, if the mass distribution is
known a priori, it becomes, at least in principle, a powerful probe of gravity
itself. Lensing observations are a more powerful tool than dynamical
measurements because they allow measurements of the gravitational field far
away from visible matter. For example, modified Newtonian dynamics (MOND) has
no relativistic extension, and so makes no firm lensing predictions, but
galaxy-galaxy lensing data can be used to empirically the deflection law of a
point-mass. MONDian lensing is consistent with general relativity, in so far as
the deflection experienced by a photon is twice that experienced by a massive
particle moving at the speed of light. With the deflection law in place and no
invisible matter, MOND can be tested wherever lensing is observed. The
implications are that either MONDian lensing is completely non-linear or that
MOND is not an accurate description of the universe.
