How to use vuforia AR Camera for Zoom

I have tried normal unity pinch gesture and it’s working but when i try this for AR Camera it’s not working. How can we use AR Camera instead of normal unity camera

using UnityEngine;
public class PinchZoom : MonoBehaviour
    public float perspectiveZoomSpeed = 0.5f;        // The rate of change of the field of view in perspective mode.
    public float orthoZoomSpeed = 0.5f;        // The rate of change of the orthographic size in orthographic mode.
    void Update()
        // If there are two touches on the device...
        if (Input.touchCount == 2)
            // Store both touches.
            Touch touchZero = Input.GetTouch(0);
            Touch touchOne = Input.GetTouch(1);
            // Find the position in the previous frame of each touch.
            Vector2 touchZeroPrevPos = touchZero.position - touchZero.deltaPosition;
            Vector2 touchOnePrevPos = touchOne.position - touchOne.deltaPosition;
            // Find the magnitude of the vector (the distance) between the touches in each frame.
            float prevTouchDeltaMag = (touchZeroPrevPos - touchOnePrevPos).magnitude;
            float touchDeltaMag = (touchZero.position - touchOne.position).magnitude;
            // Find the difference in the distances between each frame.
            float deltaMagnitudeDiff = prevTouchDeltaMag - touchDeltaMag;
            // If the camera is orthographic...
            if (camera.isOrthoGraphic)
                // ... change the orthographic size based on the change in distance between the touches.
                camera.orthographicSize += deltaMagnitudeDiff * orthoZoomSpeed;
                // Make sure the orthographic size never drops below zero.
                camera.orthographicSize = Mathf.Max(camera.orthographicSize, 0.1f);
                // Otherwise change the field of view based on the change in distance between the touches.
                camera.fieldOfView += deltaMagnitudeDiff * perspectiveZoomSpeed;
                // Clamp the field of view to make sure it's between 0 and 180.
                camera.fieldOfView = Mathf.Clamp(camera.fieldOfView, 0.1f, 179.9f);

It seems not to be possible: