  When light is transmitted through optically inhomogeneous and anisotropic
media the spatial distribution of light can be modified according to its input
polarization state. A complete analysis of this process, based on the paraxial
approximation, is presented, and we show how it can be exploited to produce a
spin-controlled-change in the orbital angular momentum of light beams
propagating in patterned space-variant-optical-axis phase plates. We also
unveil a new effect. The development of a strong modulation in the angular
momentum change upon variation of the optical path through the phase plates.
