Using new photometric and spectroscopic data in the fields of nine strong gravitational lenses that lie in galaxy groups , we analyze the effects of both the local group environment and line-of-sight galaxies on the lens potential .
We use Monte Carlo simulations to derive the shear directly from measurements of the complex lens environment , providing the first detailed independent check of the shear obtained from lens modeling .
We account for possible tidal stripping of the group galaxies by varying the fraction of total mass apportioned between the group dark matter halo and individual group galaxies .
The environment produces an average shear of \gamma = 0.08 ( ranging from 0.02 to 0.17 ) , significant enough to affect quantities derived from lens observables .
However , the direction and magnitude of the shears do not match those obtained from lens modeling in three of the six 4-image systems in our sample ( B1422 , RXJ1131 , and WFI2033 ) .
The source of this disagreement is not clear , implying that the assumptions inherent in both the environment and lens model approaches must be reconsidered .
If only the local group environment of the lens is included , the average shear is \gamma = 0.05 ( ranging from 0.01 to 0.14 ) , indicating that line-of-sight contributions to the lens potential are not negligible .
We isolate the effects of various theoretical and observational uncertainties on our results .
Of those uncertainties , the scatter in the Faber-Jackson relation and error in the group centroid position dominate .
Future surveys of lens environments should prioritize spectroscopic sampling of both the local lens environment and objects along the line of sight , particularly those bright ( I < 21.5 ) galaxies projected within 5′ of the lens .