Both weak and strong gravitational lensing have been used to constrain cosmology, via measurements of the clustering parameter S8 with cosmic shear, and measurements of the Hubble parameter H0 and dark energy equation of state w with strong lensing time delays. However, strong lensing images are themselves weakly lensed, and this `weak lensing of strong lensing' has the potential to become a powerful cosmic probe in its own right. In this talk, I will review the so-called line-of-sight formalism which describes the weak lensing of strong lensing, show how the line-of-sight shear escapes from degeneracies with other lens model parameters, and present the first measurement of this quantity in 50 strong gravitational lenses from the SLACS catalogue.
