<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def focal(self):
 """ Get the focal length in pixels for the camera. Returns focal : (2,) float Focal length in pixels """

        if self._focal is None:
            # calculate focal length from FOV
            focal = [(px / 2.0) / np.tan(np.radians(fov / 2.0))
                     for px, fov in zip(self._resolution, self.fov)]
            # store as correct dtype
            self._focal = np.asanyarray(focal, dtype=np.float64)

        return self._focal