Problem with Instantiating object at Raycast hit point.

Hey everyone, I'm having problems with raycast. I'm trying to instantiate a gameobject at the position where the raycast hits. The raycast always sends from the centre of the screen. Whenever I test this out without maximizing the screen, it works perfectly but when maximized, the object instantiates at only a small distance from the player. I've narrowed it down to the code where I find the centre of the screen, but I'm unsre how this is having an effect on the raycast distance. Here's my code.

     if(Input.GetMouseButtonDown(1)){
         Ray ray = Camera.main.ScreenPointToRay(new Vector3((Screen.height * 0.5f), (Screen.width * 0.5f), 0);
         RaycastHit hit;
         if (Physics.Raycast(ray, out hit, 5000)){
             object = Instantiate(object, hit.point, Quaternion.identity) as GameObject;
         }

Thanks for your help!