Monocular telescope having a focusing unit whose mechanical sensitivity can be varied

A monocular telescope having a constant length includes a lens mounted in a tube and interchangeable eyepieces to vary the magnification. Focusing is performed using a positioning member to move a focusing unit axially in the tube. The rate of focus adjustment is adaptable to the selected magnification due to the provision of a transmission gear having different diameter gear wheels arranged along the positioning member. The transmission gear can be operatively coupled to the interchangeable eyepiece.