How to make camera zooming with a range? [C#]

Hey guys,

I am a very new user of Unity, but for my project at college I need to make a camera zooming with a range, so when some key button is pressed the camera zooms out for a certain range and then stops in zooming. But when the button is released, the camera moves back to the initial distance from the character.

I tried to make it, camera zooms out , but it doesn't stop, it's continuously moving backwards.

 public GameObject Target;
 public float OffsetY = 2.0f;
 public float OffsetZ = 0.0f;
 public float Offset;
 public float OffsetZMax = 10.0f;
 private bool ZoomOn = false;
 public float SmoothTime = 0.075f;
 
 // Use this for initialization
 void Start () {
     
 
     }
     
     // Update is called once per frame
     void Update () {
 
 
     Vector3 destination = new Vector3(Target.transform.position.x, Target.transform.position.y + OffsetY, transform.position.z-Offset);
     Vector3 velocity = Vector3.zero;
     transform.position = Vector3.SmoothDamp(transform.position, destination, ref velocity, SmoothTime);
 
     if (Input.GetKey(KeyCode.F))
     {
         //OffsetZ += 1;
         
     }
     if (OffsetZ >= OffsetZMax)
     {
         OffsetZ = OffsetZMax;
 
     }
     Offset = OffsetZ;   
 }

I don't know to add the range to the zoom and zooming back to the initial position thing. Maybe someone knows how to do it? Or at least where I can find the solution for it? Thank you!