Focal length (from Latin: focus - hearth, fire)
~~ of the optical system: the distance from the main focus to the nearest main point of this system (the main points are the points of intersection with the optical axis of the main planes, the mutual image of which is given by the optical system in full size). Affects the magnification, aperture of the optical system. Distinguish front focal length, related to the space of objects and the back focal lengthimage space.
Complex optical systems have an equivalent focal length, depending on the focal lengths of each element of the system. The values of the focal lengths (front and back) are inversely proportional to the corresponding refractive indices of the mediums of the spaces of objects and images.