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 ...

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