  Using a sample of almost 7000 strong MgII absorbers with 0.4 < z < 2.2
detected in the SDSS DR4 dataset, we investigate the gravitational lensing and
dust extinction effects they induce on background quasars. After carefully
quantifying several selection biases, we isolate the reddening effects as a
function of redshift and absorber rest equivalent width, W_0. We find the
amount of dust to increase with cosmic time as (1+z)^(-1.1 +/- 0.4), following
the evolution of cosmic star density or integrated star formation rate. We
measure the reddening effects over a factor 30 in E(B-V) and we find the dust
column density to be proportional to W_0^(1.9 +/- 0.2), which provides an
important scaling for theoretical modeling of metal absorbers. We also measure
the dust-to-metals ratio and find it similar to that of the Milky Way.
  In contrast to previous studies, we do not detect any gravitational
magnification by MgII systems. We measure the upper limit \mu<1.10 and discuss
the origin of the discrepancy. Finally, we estimate the fraction of absorbers
missed due to extinction effects and show that it rises from 1 to 50% in the
range 1<W_0<6 Angstrom. We parametrize this effect and provide a correction for
recovering the intrinsic distribution of absorber rest equivalent widths.
