Features & details Male thread - M48*0.75. Female thread - M48*0.75, eliminates a range of focusing problems. The extension tube has a M48*0.75 interior thread and a M48*0.75 exterior thread for elegantly solving spacing problems in astrophotography. Use this extension tube for getting the correct image scale or the exact spacing required for reducers or field flatteners. All metal construction and anodizing aluminium process, durable for long time use. The required spacing can be achieved by combining several extenders of differing lengths,you can use 1 piece, 2 pieces , 3 pieces or 4 pieces of the extension rings as you like.