Lenses

RE this thread

http://forum.arduino.cc/index.php?topic=372097.15

I would like to use a zoom lens with this.

The problem is that it already has a crimped on lens and i do not know how to calculate the required lenses, i think i can do it for a focal plane but do not know how to compensate for the already attatched one.

EDIT

I was wondering if a defocussed beam expander in reverse might do the trick.