An apparatus includes an adjustment knob, an inner locking ring, and an outer locking ring threadably coupled to the inner locking ring. The adjustment knob includes at least one tooth protruding radially inward from an inner peripheral edge of the adjustment knob. The inner locking ring includes at least one tooth protruding radially outward from the inner locking ring and configured to engage the at least one tooth of the adjustment knob to prevent rotational movement of the adjustment knob. The outer locking ring is configured to move the inner locking ring in a first axial direction toward the adjustment knob when the outer locking ring is rotated in a first rotatable direction. The outer locking ring is also configured to move the inner locking ring in an opposite second axial direction away from the adjustment knob when the outer locking ring is rotated in an opposite second rotatable direction.