On the inverted microscope, different types of consumables such as Petri dish and perforated plates are often used. The thickness of the bottom is different, which will change the light passing. At this time, the objective lens with calibration ring function must be used, and its middle part is equipped with a ring mounted adjustment ring. When the adjustment ring is rotated, the distance between the lens groups in the objective lens can be adjusted, so as to correct the aberration caused by the non-standard thickness of the cover glass (Petri dish) (1.2mm for conventional Petri dish, 0.17mm for cover glass). The correct usage method is to adjust the calibration ring to the standard value of 1.2mm and focus on the sample. Adjust the correction ring to the right half grid, and then focus on the sample. If the image effect improves, adjust it to the right and then focus again, and vice versa.