ABSTRACT
Several important relations in thick optical systems may be found with only the definitions of effective focal length and Lagrange’s theorem. This chapter considers the simplest case of a system of two thin lenses separated by a finite distance. The Lagrange invariant in a system of thin lenses may adopt another form in terms of the power P of the lenses and the slopes of the meridional and principal rays. Hence, if the object and index media are the same, the Lagrange invariant remains constant with this stop shift. Using again Lagrange’s theorem, we may also derive a relation describing the effect of a shifting in the object position and its corresponding shifting in the image position.
