## 1 Introduction  
\"Mirrorless\" is a familiar term for general users and consumers of cameras. Its official name is a compact system camera (CSC) and is also known as a mirrorless interchangeable lens camera (MILC) [1]. The CSC is a form of the digital single lens reflex (DSLR) in which the reflective mirror and the optical view finder are removed [2]. As a result, the volume and weight are dramatically reduced when compared to conventional DSLR type cameras. Further, no mirror shock and black out are caused by DSLR because there is no reflection mirror movement inside the camera [3]. As CSCs did not have an auto focus (AF) sensor at their introduction, the camera essentially used contrast detection auto focus (contrast AF) [4]. Hence, AF speed was significantly slower when compared to contrast AF. The phase difference AF system calculates the difference between the reference phase and the current phase and determines the amount of movement required to achieve optimum focus, while the contrast AF moves the lens group by small amounts to achieve the same [5]. As the method determines the contrast by the image sensor and finds the optimum focus, AF was not rapid. However, the latest technological enhancements to improve AF speed, such as hybrid AF using the phase contrast AF method and contrast AF method simultaneously, dual pixel CMOS AF, and depth from defocus (DFD) AF method, have improved to the point that this deficiency is no longer an issue when compared to DSLR [6]. At the same time, developments in the area of image sensors, progressing from the M4/3(micro four thirds) sensor to the full-frame sensor, have been steadily advancing. In pursuit of the high-quality achieved by the full-frame DSLR in CSC, a CSC that adopted an APS-C (advanced photo system, type-C) sensor was introduced for the first time in Korea in 2010 and a CSC that adopted a full-frame sensor was released in Japan in 2013 [7; 8]. Meanwhile, the digital camera industry in 2017 was equipped with a 4K UHD (ultra-high definition) 60 frame/s video recording function [9]. The movie recording function began to be used when the camera sensor was replaced with a digital sensor. However, problems such as AF speed and AF noise, which were detrimental in video recording, were gradually solved through technological advancements. Specifically, by integrating phase detection AF and contrast AF, as described above, to compensate for the respective AF methods' shortcomings, the speed and accuracy of AF in CSC were improved. The phase detection AF method's drawback is that the actual focal position does not match due to AF computation movement [10]. Therefore, if it is supplemented with contrast AF, that focuses accurately, it is possible to focus on fast-moving objects even in dark surroundings. In addition, the compensation method with phase difference AF or DFD AF was used to determine the focusing direction. Contrast AF, that can focus precisely, is essential for any AF method. Additionally, a driving source capable of moving the AF group constantly and quickly is required. For this purpose, a stepper motor (STM), that has a higher precision, is used as a driving source for moving the lens. However, STM is unable to generate fast AF when the focusing group is heavy because the torque is not large [11]. Hence, when the STM is used as a driving source, the focusing group must be lightened for optical system design. In this study, the AF group designed a very wide angle optical system with a lightweight internal focus system. The inner focus system is a method in which the inner lens group moves and focuses while the first and last groups of the optical system are stationary. The floating system is a method in which two or more arbitrary lens groups move and focus. The 24 mm field of view is also included in the product range of major camera manufacturers, with field angles typically used for outdoor scenes or celestial objects.