Nodal point
The optimal turning point of the camera is the so called Nodal Point, this is the point of intersection between the optical axes and the main surface of the object. When a beam of light hits in a certain angle on the picture side nodal point, the beam of light will leave the objectside nodal point under the same angle. A good description for this point could also be "optical centre". The nodal point is impotant with taking panorama pictures.
