  Despite its mathematical complexity, the multiple gravitational lens can be
studied in detail in every situation where a perturbative approach is possible.
In this paper, we examine the caustics of a system with a lens very far from
the others with respect to their Einstein radii, and a system where mutual
distances between lenses are small compared to the Einstein radius of the total
mass. Finally we review the case of a planetary system adding some new
information (area of caustics, duality and higher order terms).

Astrophysics