Because the laser power density has a great influence on the cutting speed, the selection of lens focal length is an important issue. After the laser beam is focused, the spot size is proportional to the focal length of the lens. After the laser beam is focused by a short focal length lens, the spot size is very small, and the power density at the focal point is very high, which is very beneficial to material cutting; However, its disadvantages are that the depth of focus is very short and the adjustment margin is small. It is generally suitable for high-speed cutting of thin materials. Because the long focal length lens has a wide focal depth, as long as it has sufficient power density, it is more suitable for cutting thick workpiece.

After determining which focal length lens to use, the relative position of the focus and the workpiece surface is particularly important to ensure the cutting quality. Due to the high power density at the focus, in most cases, the focus position is just on the surface of the workpiece or slightly below the surface when cutting. During the whole cutting process, it is an important condition to ensure that the relative position of the focus and the workpiece is constant to obtain stable cutting quality. Sometimes, the lens is heated due to poor cooling during operation, resulting in changes in focal length, which requires timely adjustment of the focus position.

When the focus is in a better position, the slit is smaller and the efficiency is higher. A better cutting speed can obtain better cutting results. In most applications, the beam focus is adjusted to just below the nozzle. The distance between nozzle and workpiece surface is generally about 1.5mm.