How to determine phase and amplitude after light propagation through a lens?

[newshurik]

Hello,
Given a 3D gaussian beam incident on a lens at an angle relative to the optic axis, with phase and amplitude on the incident surface known, what is the simplest way to find the phase and the amplitude after propagation through the lens?

Sasha

 

[babemagnetics]

You would probably get more help if you put this in the homework section.