Nodal point

Nodal point in an optical system, e.g. camera lens, is a point located on lens axis, where light rays intersect. This point is used when lens focal length is determined, and it is also at this point that we find a lens iris.

Exact location of a nodal point in a camera lens became important mostly with advances made in computer graphics: QTVR (Quick Time VR) and Computer Generated 3d Visual Effects (especially in processes involving match moving, camera matching, image based modeling, motion matching, etc.) as both rely on camera rotation around it's nodal point. In classic cinematography, this type of camera orientation is preferrable as it allows for a more pleasant looking rotation without apparent change of camera position. Panning a camera around a point located behind the nodal point and usually around the base of the camera, where camera is mounted on a tripod head, will usually result in a pan or tilt which also includes some shift due to change of the camera position as well as it's orientation. This gives a less pleasant look in traditional cinematography as there will be a noticable parallax as camera is panning. In the world of Computer graphics, this problem may render a shot or set of images useless.

Since nodal point is the point of reference when determining lens focal length, it is possible to extract information on nodal point location based on the distance from the film plane towards the center of the lens. E.g. for a 50mm lens, nodal point will be 50mm in front of the film plane.