The lens equation is derived in the framework of metric tensor theories of gravity. The effects of the gravitomagnetic field and of post-post-Newtonian corrections are evaluated for relevant astrophysical systems. Two main applications of the lens equation to cosmology are also discussed. The luminosity distance-redshift relation is corrected for the effect of gravitational lensing; observations of type Ia supernovae are considered. Then, depletion curves and giant luminous arcs in clusters of galaxies are explored as tools to characterize the dark energy content of the universe.