Optical Design of a 4x Zoom Lens with a Stable External Entrance Pupil and Internal Stop

Zoom lens with stationary external entrance pupil and internal stop is a type of special optical system that can be used in cascaded optics to meet the requirements of long focal length or variable magnification. We proposed a 4x zoom lens to improve the pupil walking limitation observed in the conventional design. Varifocal- and pupil-stable differential equations are presented and a paraxial design of the lens with two moving parts was developed. Moreover, the zoom lens, which functions in the visible waveband 500 nm similar to 750 nm, is designed using seven types of common optical glasses, has a constant f-number of 10 and focal range of 100 mm similar to 400 mm, and achieves pupil walking in the range -3.9 mm to +4.3 mm. The results demonstrate that the design had good image quality and tolerance characteristics. Owing to the limited pupil walking and zoom capability, the scheme is of considerable interest for application in electrical optical systems.