Select the correct adapter for your head. Then select the 16mm lens tube or lens tube kit for your Adapter. For the longer K series heads we have a 16.15mm Adapter Here is a link to the 16mm lens tube and kit. https://american-photonics.myshopify.com/collections/mark-and-engrave-co2-and-fiber-laser/products/16mm-diameter-lens-tube-with-znse-focus-lens-or-3pc-lens-kit-alignment-tool Here is a link to the 16.15mm lens tube and kit. https://american-photonics.myshopify.com/collections/mark-and-engrave-co2-and-fiber-laser/products/16mm-diameter-lens-tube-with-znse-focus-lens-with-or-without-alignment-tool Here is a link to the 20mm lens tube and kit.   https://american-photonics.myshopify.com/collections/mark-and-engrave-co2-and-fiber-laser/products/20mm-diameter-lens-tubes-with-znse-focus-lens-7pc-kit-w-alignment-tool Easy clean lens assembly. Clean your lens without touching it. Lens is suspended in the tube by glue for stress free optical focusing APC holds a +/- .15mm(.006") tolerance on focal length on all of our lenses. The the distance to from the front of the nozzle to the perfect focal point is 10.5mm (the same as a AAA battery) Please allow us to mount your lens for you to the highest standards, in the best possible environment. Your browser does not support our video.