Node (optics)

The nodes K (on the object side) and K '(on the image side) are the points on the optical axis of an optical system for which the following applies: A ray in the object space that passes through K (or aims at K, but first at the first surface of the system is broken), goes in the image space , i.e. after passing through the system, through K 'and maintains its direction, ie the object and image space ray are parallel.

If the refractive indices of the media before the first and after the last surface of the system match (mostly there is air there), then the nodes K, K 'coincide with the main points H, H' (see main plane ). Then the focal length of the object and the image are also the same apart from the sign . An exception are e.g. B. Underwater lenses where there is water in front of the first surface.