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);
             }
             else
             {
                 // 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);
             }
         }
     }
 }