So, first of all, we need to know what focal length is, which is the distance from the center of the convex lens to the point where light converges. The length between these points is the focal length, and the convergence position is the focal point. Additionally, the focal length of any convex lens is fixed, and the laser can only achieve maximum power and effect at the focal point. Therefore, if the position of object B changes, So the entire convex lens also needs to move relative to ensure that the focus is always on the surface of B, which is because we only use the focus, so there is no need to take the definition of focal length so seriously. Just make sure to use the same height value, so both dashed lines representing the focal length on the way can be used. You can also find other positions as starting points. So, after knowing what the focal length is, we only need to adjust the lifting and lowering of the convex lens. So, for the convex lens of the laser marking machine, it is to be combined into a field mirror and installed on the galvanometer