Handbook of Optical Design, 3rd Edition

3	Systems of Several Lenses and Thick Lenses

3.1    FOCAL LENGTH AND POWER OF A LENS SYSTEM

The lateral magnification of a thick centered optical system in air, using the Lagrange theorem in Equation 1.64 and the definition of lateral magnification for a distant object, is given by

$m=\frac{u_1}{u_k}=\frac{y_1}{l_1{u^{\prime }}_k},$

(3.1)

assuming that the object and image media are the same (typically air).

The effective focal length of a thick lens or system of lenses is defined by

$F^{\prime }=-\frac{y_1}{{u^{\prime }}_k};$

(3.2)

hence, the lateral magnification with a distant object depends only on the effective focal length F, independently of the particular lens configuration. With this definition, we may see from Figure 3.1 that the effective focal length is the distance from the focal plane ...