Abstract: A television camera objective has a driving mechanism for adjusting components of its optical system and is adapted selectively replaceably to receive either a servo control driving unit or a manual control driving unit. Either type of driving unit has a driving shaft that engages with a rotary transmission shaft that remains in place on the driving mechanism. To prevent the transmission shaft from rotating in the absence of a driving unit, the transmission shaft has a flange against which a sliding sleeve is spring urged into braking frictional engagement. The introduction of the driving unit pushes back this sleeve to disengage the frictional surfaces and release the transmission shaft for rotation. In addition, the sleeve can be manually pulled back and rotated to lock it in a disengaged position, so as intentionally to free the transmission shaft for rotation in the absence of a driving unit.