  We propose a method for detecting gravitational magnification of distant
sources, like quasars, due to absorber systems detected in their spectra. We
first motivate the use of metal absorption lines rather than Lyman-alpha lines,
then we show how to relate the observed moments of the source magnitude
distribution to the mass distribution of absorbers. In order to illustrate the
feasibility of the method, we use a simple model to estimate the amplitude of
the effect expected for MgII absorption lines, and show that their lensing
signal might already be detectable in large surveys like the SDSS. Our model
suggests that quasars behind strong MgII absorbers are in average brightened by
-0.05 to -0.2 magnitude due to magnification. One must therefore revisit the
claim that, in magnitude limited surveys, quasars with strong absorbers tend to
be missed due to extinction effects. In addition to constraining the mass of
absorber systems, applying our method will allow for the quantification of this
bias.
