Average magnification effect of clumping of matter
  The aim of this paper is to re-examine the question of the average magnification in a universe with some inhomogeneously distributed matter. We present an analytic proof, valid under rather general conditions, including clumps of any shape and size, and strong lensing, that so long as the clumps are uncorrelated the average \emph{reciprocal} magnification (in one of several possible senses) is precisely the same as in a homogeneous universe with equal mean density. From this result, we also show that a similar statement can be made about one definition of average \emph{direct} magnification. We discuss, in the context of observations of discrete and extended sources, the physical significance of the various different measures of magnification and the circumstances in which they are appropriate. 
T.W.B. Kibble and Richard Lieu