Using Weak Lensing to find Halo Masses
  Since the strength of weak gravitational lensing is proportional to the mass along the line of sight, it might be possible to use lensing data to find the masses of individual dark matter clusters. Unfortunately, the effect on the lensing field of other matter along the line of sight is substantial. We investigate to what extent we can correct for these projection effects if we have additional information about the most massive halos along the line of sight from deep optical data. We conclude that unless we know the masses and positions of halos down to a very low mass, we can only correct for a small part of the line-of-sight projection, which makes it very hard to get accurate mass estimates of individual halos from lensing data. 
Roland de Putter, Martin White