Because of some force majeure reasons, such as processing errors, thermal expansion and cold contraction, even the camera of the same model will have certain problems in the flange distance of its bayonet. This difference will lead to problems such as inaccurate lens calibration and out of focus in actual use, which will greatly affect the use effect. Usually, users will compensate the flange distance by adding or reducing gaskets at the bottom of the bayonet. This gasket is used to fine-tune the lens optical path, which is very important and a meticulous work. This method generally needs to disassemble and assemble the lens for more than three times, and add and subtract gaskets to confirm the effect (equivalent to correcting the lens quality in the final inspection stage). This is sloppy, and the quality of the lens in your hand depends on luck.