<title>Simple Quasi-Zoom With Stable Pupils</title>

Abstract

Abstract In conventional zoom systems with the aperture stop located to the rear of the moving components, the entrance pupil position varies greatly during zooming. A simple system of three positive elements, is described which can be used as a dual focal length quasi- zoom lens, which for a rear-positioned aperture stop, has a constant entrance pupil at the zoom extremes. The system may also be employed as a true zoom with limited excursion of the entrance pupil between the zoom extremes. The system is useful as a zoom relay stage in a periscope or where a prism or mirror is located at the entrance pupil to scan the line of sight.IntroductionIn conventional zoom systems the movements of the various lens elements are controlled such that the image plane remains in a fixed position as the magnification varies. The entrance and exit pupils are usually not controlled and for a fixed aperture stop, the position of the entrance pupil varies as the magnification varies. This variation in position is undesirable in many cases, for example in an image relay stage of a periscope or in a system employing a prism or mirror to provide line of sight elevation and azimuth control. Hopkins has shown that in a true zoom system (i.e. one which provides continuous variation in magnification), three separate moving elements are necessary for constancy of both image and pupil positions. It is possible, however, to achieve constant image and pupil positions at the zoom extremes with a single moving element. The system is a simple dual power optical system with stationary image and pupils. By moving a second element we have a true zoom system with quasi stationary pupils.