  Modifications to GR generically predict time and scale-dependent effects
which may be probed by observations of strong lensing by galaxies. Measurements
of the stellar velocity dispersion determine the dynamical mass whereas
measurements of the Einstein radius determine the lensing mass. In GR these two
masses are equal; in alternative gravity theories they may not be. Using
measurements of the stellar velocity dispersion and strong lensing around
galaxies from the Sloan Lens ACS (SLACS) survey we place constraints on lensing
in modified gravity theories and extend previous studies by applying this data
to explore its dependence on various properties of the lens such as the lens
redshift or mass and thereby constrain scalar-tensor, f(R) gravity theories,
and generic parameterizations of deviations from GR. Besides applying the
observations to these specific gravity theories, the data places a constraint
on a generic dependence of modifications to GR on the lens mass and redshift.
At the 68% confidence level we find that the ratio between the lensing and
dynamical masses can only vary by less then 50% over a mass range for the lens
galaxies of 1E12 < M/Msun < 1E14 and less than 40% over the redshift range 0.06
< z < 0.36.
