Camera Drag and Zoom with Mouse in Unity 3D

The script is easy and descriptive, it is the result of some searching and modification.

• Press the left mouse button and drag to move, and use mouse scroll wheel to zoom in and out.
• The zooming code can work for both orthogonal and perspective but I've been using and testing it in orthogonal mode. 
• Modify the public values to fit in your game.

** Just drag the script to your camera and it will work! **

C# code
[CameraDragZoom.cs]

using UnityEngine;
using System.Collections;

public class CameraDragZoom : MonoBehaviour {
    
    public float dragSpeed = -10;
    public int minX = -892;
    public int maxX = 1111;
    public int minZ = -880;
    public int maxZ = 1145;
    
    public int bottomMargin = 80; // if you have some icons at the bottom (like an RPG game) this will help preventing the drag action at the bottom
    
    public float orthZoomStep = 10.0f;
    public int orthZoomMaxSize = 500;
    public int orthZoomMinSize = 300;
    
    private bool orthographicView = true;
    private Vector3 dragOrigin;
    
    // Update is called once per frame
    void Update () {
        moveCamera();
        zoomCamera();
    }
    
    void moveCamera()
    {
        if (Input.GetMouseButtonDown(0))
        {    
            dragOrigin = Input.mousePosition;
            return;
        }

        if (!Input.GetMouseButton(0)) return;
        
        if(dragOrigin.y <= bottomMargin) return;
        
        Vector3 pos = Camera.main.ScreenToViewportPoint(Input.mousePosition - dragOrigin);
        Vector3 move = new Vector3(pos.x * dragSpeed, 0, pos.y * dragSpeed);
                
        if(move.x > 0)
        {
            if(!isWithinRightBorder())
                move.x =0;
        }
        else
        {
            if(!isWithinLeftBorder())
                move.x=0;
        }
        
        if(move.z > 0)
        {
            if(!isWithinTopBorder())
                move.z=0;
        }
        else
        {
            if(!isWithinBottomBorder())
                move.z=0;
        }
            
        
        transform.Translate(move, Space.World);
    }
    
    void zoomCamera()
    {
        if(!isWithinBorders())
            return;
        
        // zoom out
        if (Input.GetAxis("Mouse ScrollWheel") <0)
        {
            if(orthographicView)
            {
                if (Camera.main.orthographicSize <=orthZoomMaxSize)
                    Camera.main.orthographicSize += orthZoomStep;
            }
            else
            {
                if (Camera.main.fieldOfView<=150)
                       Camera.main.fieldOfView +=5;
            }
        }
        // zoom in
        if (Input.GetAxis("Mouse ScrollWheel") > 0)
           {
            if(orthographicView)
            {
                if (Camera.main.orthographicSize >= orthZoomMinSize)
                     Camera.main.orthographicSize -= orthZoomStep;            
            }
            else
            {
                if (Camera.main.fieldOfView>2)
                    Camera.main.fieldOfView -=5;
            }
           }
    }
    
    bool isWithinBorders()
    {
        return ( isWithinLeftBorder() && isWithinBottomBorder() && isWithinRightBorder() && isWithinTopBorder() );
    }
    
    bool isWithinLeftBorder()
    {
        Vector3 currentTopLeftGlobal = Camera.main.ScreenToWorldPoint(new Vector3(0,0,0));
        if(currentTopLeftGlobal.x > minX)
            return true;
        else
            return false;
        
    }
    
    bool isWithinRightBorder()
    {
        Vector3 currentBottomRightGlobal = Camera.main.ScreenToWorldPoint(new Vector3(Screen.width,0,0));
        if(currentBottomRightGlobal.x < maxX)
            return true;
        else
            return false;
    }
    
    bool isWithinTopBorder()
    {
        Vector3 currentTopLeftGlobal = Camera.main.ScreenToWorldPoint(new Vector3(0,Screen.height,0));
        if(currentTopLeftGlobal.z < maxZ)
            return true;
        else
            return false;
    }
    
    bool isWithinBottomBorder()
    {
        Vector3 currentBottomRightGlobal = Camera.main.ScreenToWorldPoint(new Vector3(Screen.width,0,0));
        if(currentBottomRightGlobal.z > minZ)
            return true;
        else
            return false;
    }
}