How to do a Zoom in/out with Cardboard SDK for Android with Unity

Question

I'm building a Cardboard app with Unity3D that should be able to Zoom in/out with the Magnetic Trigger. In the StereoController.cs changed the MatchMonoFOV=1 and MatchByZoom=1. Then i created a script with this method just for Zooming In:

public void TriggerStart()
{
    //This counts the number of active cameras and set the FOV to 40
        for (int i = 0; i < Camera.allCamerasCount; i++)
        {
                Camera.allCameras[i].fieldOfView = 40;
        }
}

Unfortunately, this works only in UnityEditor but not with the Headset. Every kind of advice will be really appreciated!