Two types of lenses go into the head set piece. One is the cover glass or windshield glass for protection. The second is the objective lens. The objective lens is glued to the small pipe with the iris in the front, and then glued into the head set piece. Following this, an image bundle with another lens glued onto it will be inserted into the pipe as well. These lenses are very small in size. During the cleaning and assembly, you must make sure there are no dust particles on the lens or air bubbles in the glue. This pipe which contains the lens and the image bundle will give you the focus – being either close focus or far focus.