We can move an entrance pupil of a zoom lens from on-axis to off-axis by decentering lens groups of a zoom lens. This decentering zoom lens makes an objective lens of a stereomicroscope small. As a result, we can develop a stereomicroscope with high magnification and high resolution.
A stereomicroscope can stereoscopically observe an object with protrusions and recesses as if the object were viewed by both eyes. Such stereomicroscopes use an optical system to create two slightly different viewing angles of an object. The different images are both enlarged and viewed through two eyepieces. Each of the observation optical systems includes a variable magnification mechanism which is called a zoom lens system. In recent years, a demand for stereomicroscopes that can observe a wide variable magnification range has been increasing along with the diversification of applications. However, there are no stereomicroscope zoom lenses with both a high resolution and a large zoom ratio. We developed the “Perfect Zoom System” which can reduce the light flux diameter going through the objective lens in the low-power state. In the “Perfect Zoom System”, the zoom lens groups move along not only the optical axes but also perpendicular to the axes. Therefore, the objective lens could be made smaller by decentering the G2 and G3 group lenses in zoom lens group. We achieved a high resolution and a zoom ratio of 25:1.