ortographic camera smooth follow with zoom

at http://answers.unity3d.com/questions/29183/2d-camera-smooth-follow.html i found pretty simplet and really nice working script for camera follow

 var dampTime : float = 0.3; //offset from the viewport center to fix damping
 private var velocity = Vector3.zero;
 var target : Transform;
  
 function Update() {
     if(target) {
         var point : Vector3 = camera.WorldToViewportPoint(target.position);
         var delta : Vector3 = target.position - camera.ViewportToWorldPoint(Vector3(0.5, 0.5, point.z));
         var destination : Vector3 = transform.position + delta;
  
        // Set this to the Y position you want the camera locked to
         destination.y = 0; 
  
         transform.position = Vector3.SmoothDamp(transform.position, destination, velocity, dampTime);
     }
 }

it's almost perfect i just need put camera.ortographic size there to be able to zoom simultaneously so i tried to change Z coordinate to ortographic size and i can't findout why it's nor working (actually it's zooming out all the time)

here's my current script:

     private void Update()
     {
         cameraPosition = new Vector3(target.x, target.y, camera.orthographicSize);
 
         Vector3 difference = camera.ViewportToWorldPoint(new Vector3(0.5f, 0.5f, 0f)) + Vector3.forward*camera.orthographicSize;
         Vector3 delta = cameraPosition - difference;
         Vector3 destinationMovement = cameraPosition + delta;
 
         cameraPosition = Vector3.SmoothDamp(cameraPosition, destinationMovement, ref velocity, currentBoundaries.dampTimeMovement);
         transform.position = new Vector3(cameraPosition.x, cameraPosition.y, transform.position.z);
         camera.orthographicSize = cameraPosition.z;
     }