In accordance with one embodiment of the present invention, a positioning device comprises a microscope assembly and a lens positioning assembly. The lens positioning assembly comprises a lens support subassembly and a bi-directional positioning guide. The lens support subassembly comprises a translating rod, while the positioning guide comprises a coiled spring. The coiled spring is canted relative to a longitudinal axis of the rod. The inner periphery of the coiled spring engages in a spring loaded state an outer periphery of the rod such that the coiled spring provides a frictional force about the outer periphery of the rod. This frictional force provided by the coiled spring about the outer periphery of the rod is sufficient to hold the rod within the inner periphery, but permit infinitely variable bi-directional movement of the rod through the inner periphery under an applied force. Additional embodiments are disclosed and claimed.