Focus beam
In geometrical optics , the focal point ray is a ray of light that passes through a focal point of a lens . Focal rays are always refracted so that they leave the lens parallel to the optical axis - or vice versa: rays that are parallel to the opt. Axis hit the lens then leave it as a focal ray.
On the other hand, the focal point beam is a graphic aid to determine the beam path of an optical system . The focal point beam is drawn away from the object through the focal point to the associated main plane . From the point of intersection with the focal plane , the beam is continued parallel to the optical axis . From the point of intersection of the focal beam with the parallel beam and the central beam, the position of the image can be determined very comfortably.