介绍了一种可实现高变倍比变焦的结构形式，该结构形式由变焦物镜组和变焦二次成像组两部分组成，系统的变倍比是前后两组变焦系统变倍比的乘积。系统中间像面处像面高度和数值孔径在变焦过程中可变，在短焦位置，中间像面高度大，数值孔径小；在长焦位置，中间像面高度小，数值孔径大，这种结构形式有利于系统的像差校正。与传统变焦距系统相比，该变焦距结构形式较容易实现高变倍比变焦。利用该结构形式设计了一套80倍制冷型长波红外变焦距系统，系统全焦距范围内调制传递函数(MTF)设计值在空间频率14 lp/mm处大于0.26，表明该结构形式可用于实现高变倍比变焦距系统。

A high zoom-ratio zoom structure type is introduced, which is composed of zoom imaging group and zoom reimaging group. The zoom ratio of system is the product of the two zoom groups. The size and numerical aperture of the intermediate image plane can be changed in the zoom process, when system is in short focal length position, the height of the immediate image is large while the numeric aperture is small. When system is in long focal length, the height of the immediate image is small while the numeric aperture is large. This will help to correct aberrations of image. Compared with the traditional zoom system, this type of zoom system structure is easier to obtain high zoom ratio. Using this structure a 80× cooled long-wave infrared zoom system is designed. The modulation transfer function (MTF) is above 0.26 in all focal length at the spatial frequency of 14 lp/mm. The result indicates that the structure can be used to achieve high zoom ratio zoom system.