# Design of a Cell Phone Lens-Based Miniature Microscope with Configurable Magnification Ratio
## 2 Materials and Methods
### Down-Selection of Different Effective Focal Length (EFL) Objectives  
According to the concept of the switchable object lens, we purchased a series of off-the-shelf cell phone camera modules and disassembled them to obtain a series of lenses with different focal lengths (Figure 2A). As accurate camera lens information is not easy to obtain, we firstly measured the focal lengths of lenses, as shown in Figure 2B. The fixed tube lens module is composed of a camera module (Figure 2C) with a known focal length (EFL = 6 mm) and a Sony IMX258 sensor (pixel size = 1.12 \(\upmu\)m). The object lenses are connected with the module to image a 100 \(\upmu\)m wide line target (Edmund Optics). In this process, we designed an experimental system for precise adjustment of the working distance, building it with a 3D printer (Figure 2D). As for illumination, we utilized a simple  
Figure 1: Principle of a miniature cell phone microscope. (**A**) Under normal circumstances, the cell phone lens can image a large field of view on a highly flat CMOS image sensor. This means that according to the principle of reversibility of light paths, high-quality microscopic imaging can be carried out when the cell phone lens is reversed. (**B**) Switchable reversed cell phone lenses are used as switchable objectives. They couple light from the sample through the tube lens and form the image of the sample on the CMOS image sensor. At last, the cell phone will process and display microscopic images. The microscope module is outside of the cell phone.  
LED diffuse backlight in the transmission approach. The results of the imaging test are presented in Figure 3.  
The imaging test results of the six dismantled lenses are shown in Figure 3. The magnification ratio can be determined according to the line width (in pixels) in the image, the pixel size on the sensor and the nominal line width of the sample. The focal length of the dismantled lenses can be calculated based on Equation (1). As shown in Table 1, by selecting different lens modules, we can realize the switching range of magnification of \(\sim\)3.9\(\times\). In the following experiments, we selected three lenses as the objectives in the miniature microscope, namely LG G5 super wide-angle lens (FL = 1.5 mm), Huawei P20 pro standard lens (FL = 6 mm) and P20 pro wide-angle lens (FL = 4 mm). The larger focal length lens (Huawei P20 pro tele-photo) was not selected for aperture mismatch reasons, which will be discussed in the following part.  
Figure 3: Photos of 100 μm wide line target obtained by the tube lens (EFL = 6 mm) matched with different cell phone camera lenses. (**A–F**) The pictures correspond to different object lenses in turn, which are LG G5 super wide-angle lens, P20 pro wide-angle lens, P20 pro tele-photo lens, P20 pro standard lens, Mate20 wide-angle lens and Mate20 standard lens.  
Figure 2: Measurement of selected off-shelf cell phone camera lens. (**A**) The six cell phone lenses with distinct focal lengths are all obtained from disassembled cell phones of Huawei P20 pro, Mate20, and LG G5. (**B**) The focal length measurement device consists of a diffused light source, calibration plate, the lens to be measured and a camera module with known parameters. (**C**) The image of the tube lens camera module with known focal length and Sony IMX258 sensor. (**D**) The experimental system consists of a microscope module, displacement stage, calibration and light source. The frame is produced by 3D printing.