Optical design of an inner focus type telephoto lens with a large aperture

Abstract As consumer demand for high-performance lenses increases, lenses with a large aperture of F/1.2 are being developed in the auto-focus (AF) market. As the F-number decreases, the diameter of the lens increases, which increases aberration as well as the weight of the lens. Those used in photography cameras must be very small and thus have relatively weak torque. Although motors with stronger torque, such as ultrasonic motors, are available, they generate noise when the lens moves, which is fatal in filming video content. To solve this problem, a design using a floating system that moves two lens groups has also been proposed. However, this becomes more complicated in terms of mechanical and electronic control compared with the method of moving a single lens group. Therefore, in this study, we propose a novel design for an F/1.2, f = 85 mm large aperture telephoto lens that uses one AF group and consists of a single lens to lighten the moving lens group for quiet and precise focusing. Even at the maximum aperture of F/1.2, the modulation transfer function (MTF) performance was maintained at 80% in the center and 69% at the periphery. By reducing the weight of the moving lens group by using a single focusing group, the proposed system was able to operate quickly and quietly without any significant degradation in performance, even in the minimum object distance. Therefore, our results demonstrate that an internal focusing structure based on a single focusing group is feasible even in an F/1.2, with an 85 mm large-aperture prime lens.