Point node (nodes)
The nodal points of an optical system are rated cardinal points N and not are two combined points (one - point node image is the image of the other, the point node object) they are located on the optical axis, and such as to any incident Ray passing through the point node object, match an emerging Ray through the nodal point image and parallel to the incident Ray. When the optical system is located in an environment where the index of refraction of the Middle image and the environment object are identical, the nodal points are confused with the main points.
The anatomic proximity of two nodes allow their merger in a 'single nodal point' for calculations at theVisual acuity (angular resolving power). The nodal point is located near the posterior crystalloid, and we generally choose a distance of 16.7 mm between the nodal point and the fovea for calculations.
Geometric relations between the nodal points and the other cardinal points are represented in the following diagram:
In photography, the rear nodal point is one around which rotate the device if you want to take several shots without that live their position with respect to the plan focal image moves.
In the case of the human eye, the Middle image is the vitreous (n = 1336), which is different from the middle of the air (n = 1). For a standard model look, the nodal points its located on both sides of the posterior capsule of the lens, "backward" (in the sense of the propagation of light) key points P and P'