Camera zoom script is constantly zoomed

Hello i am working on a game where you have to be able to zoom by clicking the middle mouse button. I have created a script, however when i start the game it instantly zooms in and i cant zoom out

here is the script

using System.Collections;

using System.Collections.Generic;

using UnityEngine;

public class Zoom : MonoBehaviour
{

public int zoom = 20;  
public int normal = 60;
public float smooth = 5;
public Camera cam;

private bool isZoomed = false;

void Update()
{
  if (Input.GetMouseButtonDown(1))
    {
        isZoomed = true;
    }

  if (isZoomed == true)
    {
        cam.fieldOfView = Mathf.Lerp(cam.fieldOfView, zoom, Time.deltaTime * smooth);
    }
  else
    {
        cam.fieldOfView = Mathf.Lerp(cam.fieldOfView, zoom, Time.deltaTime * smooth);
    }
}

}

You are doing the same Lerp() what ever you isZoomed or not. Does not seem to be the intention.
Try removing the second Lerp, or put -zoom (negtive zoom) when isZoomed == false.