The Baader Planetarium Varilock 15 mm T-2 contains two extensions to adjustable rings in the T-2 System. You can make a determination of the preload by loosening or tightening the ring - or it can be adjusted by means of the extension tube"s millimeter scale to the desired length. Initial tension ring is included.Varilock T-2 systems are ideal tools for setting the exact focus on intervals - such as with H-alpha solar filters or image field correctors with critical focus position. A further application can be both the Varilock T-2 thread removed by unscrewing the eyepiece"s included set screws and replacing it with the included ring swallow, which is directly connected to the T-thread of Canon EOS and Nikon-mount DSLR cameras.