LT-F-series Focusing Tubes are provided to allow axial translation of one group of lenses relative to another. The focusing tube is simply a lens tube with a longer external thread and a locking nut. Provided in 1/2-, 1- and 2-inch diameters, the focusing tube screws into the internal threads of a standard lens tube and is locked in place with the lock nut. The maximum focusing range is 0.39-inch for 1/2- and 1-inch diameters and 0.89-inch for the 2-inch diameter version. A lens retaining ring is also included with each focusing tube.