LV-180 Laser Engraver/Cutter
FL15,20,25,40 – are focusing lenses. The numbers (15, 20, etc) designate the focal length of the lens in inches.
FL20 comes standard and will work for most applications.
If the user is cutting thicker materials they will want a wider angle or FL40.
If a small and finer detail is needed, use the FL15.
The image below shows 10mm acrylic and demonstrates the divergence of the laser beam. The wider angle provides a better cut in thicker material on the FL40. The FL15 will be a finer point and provide a very small detail.