[vtkusers] MouseWheel usage with vtkInteractorStyleTerrain

[pof]

Hi,
I'm using a vtkInteractorStyleTerrain, and I would like to use the 
mousewheel to do some zoom in/out towards the scene focal point, 
actually pretty much like what is obtained using a 'L' keypress.
I've tried to change the mousewheelfactor with SetMouseWheelFactor(2), 
but still nothing happen at rendering (i.e. the camera does not move).
I 've obviously missed something. Does anybody has an idea ?
Thanks
Pof