Laser cutting machine to use the laser beam energy to cut, must be the original laser beam emitted through the lens focus, in order to form a high-energy-density spot. According to Gaussian optical theory, the focus of the highest power density. The longer the focal length of the lens, the greater the focal spot, the lower the power density, but the depth of focus (focal depth is the focal spot on both sides of the diameter of 5% of the two spot spacing, also known as the effective cutting range), the operating tolerance Big. A 5-inch lens has an effective range greater than 3-inch lenses. This is why the telephoto lens is suitable for thick plate cutting, and the tracking system requires less stability of the pitch, but requires high laser output power. On the contrary, the short focal lens is only suitable for the thin plate cutting below D3. The short focus has strict requirements on the pitch stability of the tracking system, but its laser output power requirement can be greatly reduced.