One of the advantages of laser cutting is the high energy density of the beam, generally 10W/cm2.Since the energy density is inversely proportional to the area, the focal spot diameter should be as small as possible to produce a narrow slit.At the same time, the focal spot diameter is also proportional to the focal depth of the lens.The smaller the focal depth of the focusing lens, the smaller the focal spot diameter.However, if the lens is too close to the workpiece, the lens will be damaged easily. Therefore, the focal length of 5 "~7.5" (127~190mm) is widely used in the industrial application of high-power CO2 laser cutting machine.The actual focus spot diameter is between 0.1 and 0.4mm.For high quality cutting, the effective depth of focus is also related to the diameter of the lens and the material to be cut.For example, 5 lens is used to cut carbon steel, the focal depth is within the range of +2% of the focal length, that is, about 5mm.Therefore, the control focus is very important relative to the position of the material surface being cut.Concerns about cutting quality, cutting speed and other factors, in principle, 6mm metal material, focus on the surface;6mm carbon steel with focus on the surface;6mm stainless steel with focus below the surface.The specific size is determined by the experiment.