Acta Optica Sinica, Volume. 45, Issue 10, 1011004(2025)
Optical Design of Afocal Zoom Lens with High Zoom Ratio
In the medical field, surgery remains the cornerstone of treatment for most diseases. The success of surgical procedures critically depends on the clear visualization of pathological structures after dissection. Surgical microscopes, offering adjustable magnification, bright illumination, and high-definition operative views, have become indispensable tools in microsurgery. In recent years, increasing demands for broader fields of view, finer imaging precision, and improved ergonomics have led to higher requirements for zoom ratio and compactness in optical systems. These advancements primarily rely on their core component: the afocal zoom system. Despite significant progress, a persistent trade-off exists between zoom ratio and system length, making it difficult to achieve both high magnification and compact size. Most current systems achieve only a 6× zoom ratio within an 80 mm structure, failing to optimize both simultaneously. Therefore, addressing this trade-off is a critical step toward developing high-performance surgical microscopes with significant clinical value.
To meet the growing demand for high zoom ratios in surgical microscopes, we propose a design methodology for a compact, high-zoom-ratio afocal zoom system. First, an appropriate optical configuration is selected based on system specifications. Paraxial ray tracing of the first and second principal rays is used to derive the vignetting coefficient as a function of system apertures and component focal powers, revealing the relationship between vignetting and focal power distribution. This yields an initial optical layout with minimized vignetting. Subsequently, virtual stops and lens apertures are optimized to further control vignetting. Material selection and system layout are refined to correct aberrations, resulting in a system with a 20 mm clear aperture, 80 mm total length, and a continuous 10× zoom range (0.4×?4.0×). Finally, each lens barrel is aligned independently using optical design tolerances to ensure coaxial alignment and MTF compliance. Compensation lenses in the dual-barrel system are adjusted via eccentric ring rotation, leveraging optical axis offset to align binocular optical axes at infinity, meeting the imaging requirements of surgical applications.
Based on the proposed design method, a compact, mechanically compensated afocal zoom system with a high zoom ratio is designed. The system features an overall length of 80 mm and a maximum aperture of 20 mm, and adopts a ‘+--+’ configuration comprising five lens groups. Among these, the zoom group consists of a triple-cemented lens assembly with a negative optical focal length. The overall design achieves a compact and high-performance optical layout (Fig. 8). Evaluation of the image quality of the system yields the following results: the lens cam curve exhibits a smooth trend without inflection points (Fig. 10). Relative illumination is 54% at 1× magnification and exceeds 90% at higher magnifications (Fig. 7). The modulation transfer function (MTF) remains above 0.2 at 0.4×, 1.3×, and 4.0× magnifications, corresponding to full-field average values of 20, 60, and 110 lp/mm, respectively (Fig. 11). Finally, tolerance and optical axis sensitivity analyses confirm the system’s high feasibility (Table 3, Fig. 13, and Table 4).
In this paper, we propose an afocal zoom optical system for surgical microscopes, along with a method for determining focal length distribution and initial layout based on Gaussian optics. Through detailed analysis of the relationship between vignetting and optical power allocation, as well as optimization of the virtual diaphragm and lens aperture, the design effectively controls vignetting under structural length constraints. The resulting afocal zoom system achieves a tenfold continuous zoom range of 0.4× to 4.0×, with a maximum aperture of 20 mm and a total length of 80 mm. This system demonstrates excellent imaging performance, with a smooth zoom curve free of inflection points, ensuring both engineering feasibility and suitability for practical surgical microscope applications.
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Zhaorui Hu, Linjun Gu, Jiacheng Zhu, Weimin Shen. Optical Design of Afocal Zoom Lens with High Zoom Ratio[J]. Acta Optica Sinica, 2025, 45(10): 1011004
Category: Imaging Systems
Received: Jan. 21, 2025
Accepted: Mar. 28, 2025
Published Online: May. 19, 2025
The Author Email: Jiacheng Zhu (zjc@suda.edu.cn)
CSTR:32393.14.AOS250530