# Design of a Cell Phone Lens-Based Miniature Microscope with Configurable Magnification Ratio
## 4 Discussions  
The imaging tests exhibit that the proposed miniature microscope can provide micron-level resolution and large FOV microscopic imaging with minute cost and high portability. Next, the limiting factors of miniature cell phone microscopes are discussed. The first problem is whether the FOV of the miniature microscope can be enlarged. According to Equation (1), the longer the focal length of the object lens switched to, the larger the FOV for the proposed microscope is. However, for the cell phone tele-photo lens, the increase of focal length brings with it a smaller angle of imaging, since they are usually designed for the fixed size CMOS sensor. If a long focal length tele-photo cell phone lens (for example, P20 pro tele-photo, EFL = 8 mm) is used, we find the resulting image is effective only in the central circular region, which is decided by the maximum imaging angle of the tele-photo lens (Figure 8). Compared with Figure 6, we can see that the magnification ratio decreases as expected, and the effective FOV also decreases. Therefore, we abandoned the tele-photo lens as the objective. If users really demand an even larger FOV, a specially designed tele-photo lens with larger imaging angle may be required, which may increase the cost of the total system. The proposed miniature microscope is formed by connecting two cell phone lenses. The mismatch between the position and the size of the two lens apertures would naturally have a negative impact on the imaging quality of the micro-imaging system. In Table 2, the resolution is evaluated based on the central part of the image, where the influence of aperture mismatch is minimal. It can be expected that vignetting may occur in the off-axis field of view due to aperture. From previous results, we can find that the corner area of the image is obviously darkened at a low magnification ratio (Figure 6). Resolution is also influenced by this factor. When we shifted the 8 th and 9 th group lines of the resolution test chart from middle to boundary of the field of view, the aberration aggregation can be observed and the resolution capability gets worse, as the resolvable 9-1 lines become contrast-less in the corner of the image (Figure 9). For some applications, the aberration on the edge would further weaken the available field of view, even though the imaging amount in the center is acceptable, especially when scanning or stitching is needed. By shortening the distance between the two lenses in the fixture, the influence of aperture mismatch may be reduced. In addition, when the objective lens is selected, the cell phone lens with a larger aperture is preferred. Other limiting factors also include color non-uniformity of the microscope image, short working distance, etc. However, despite all these limitations, the proposed miniature cell phone microscope with switchable magnification marks the improvement of imaging performance and user operational acceptance. Figure 8: Picture of resolution test chart using P20 pro tele-photo lens (EFL = 8 mm). Theoretically, when the longer focal length lens is used as an objective lens, the magnification will be reduced and the FOV will become larger. In practice, the FOV may be limited to a smaller circle by the objective lens imaging angle limitation. More importantly, the proposed concept can be used as a miniature platform for various interesting micro-imaging research applications. In the cell culture applications, the real-time microscopic wireless observation can be realized (Figure 10A). The compact size of the microscope module makes it possible to be placed inside the temperature-controlled cell culturing tank, and the captured images can be transferred via the cell phone in a real-time manner. In addition, the low cost of the miniature microscope makes it possible to deploy such a module array to monitor an array of cell culturing plates simultaneously. This research is already in progress and has aroused great interest.