 Actually, the camera's image plane is behind the optical center of the camera's lens.However, to simplify the calculation, images are drawn in front of the optical center of the lens by f. The u-axis and v-axis of the image's coordinate system O1uv are in the same direction with x-axis and y-axis of the camera's coordinate system respectively.The origin of the image's coordinate system is located on the intersection of imaging plane and the optical axis.