CameraOrbit.cs

using UnityEngine;
using System.Collections;

public class CameraOrbit : MonoBehaviour
{
    public float angularSpeed = 180;
    public float zoomSpeed = 1;
    public float sensibility = 6;

    Vector2 prevMousePosition;
    Vector2 angularVelocity;
    float zoomVelocity;

    void Update()
    {
        var dt = Time.deltaTime;

        var mousePosition = (Vector2)Input.mousePosition;
        var targetAngularVelocity = Vector2.zero;
        var targetZoomVelocity = Input.GetAxis("Mouse ScrollWheel") * zoomSpeed;

        if (Input.GetMouseButtonDown(0))
        {
            prevMousePosition = mousePosition;
        }
        else if (Input.GetMouseButton(0))
        {
            targetAngularVelocity = (mousePosition - prevMousePosition) * (angularSpeed / (Screen.width * dt));
            prevMousePosition = mousePosition;
        }

        var exp = Mathf.Exp(-sensibility * dt);
        angularVelocity = Vector2.Lerp(targetAngularVelocity, angularVelocity, exp);
        zoomVelocity = Mathf.Lerp(targetZoomVelocity, zoomVelocity, exp);

        var xPivot = transform.parent.parent;
        var yPivot = transform.parent;
        xPivot.localRotation = Quaternion.AngleAxis(angularVelocity.x * dt,  Vector3.up   ) * xPivot.localRotation;
        yPivot.localRotation = Quaternion.AngleAxis(angularVelocity.y * dt, -Vector3.right) * yPivot.localRotation;

        transform.localPosition += Vector3.forward * zoomVelocity * dt;
    }
}