An adjustment apparatus for adjusting a setting of a sighting device, such as a riflescope or telescope, includes an adjustment knob operatively coupled to an adjustment mechanism, where rotation of the adjustment knob about a rotational axis drives the adjustment mechanism to adjust the setting of the sighting device. The adjustment apparatus further includes a lock mechanism including a first lock element fixed relative to the sighting device and a second lock element selectively movable relative to the first lock element. When the lock elements engage one another, the adjustment knob is in a locked position and restrained from rotating about the rotational axis. The adjustment apparatus may further include a lock-release mechanism operable to disengage the lock elements and allow the adjustment knob to rotate about the rotational axis.