A turret knob comprises first and second adjustment members that are both adjustably positionable about an axis of rotation, and a slip/engagement member positioned between the first and the second adjustment members along the axis of rotation. The slip-engagement member is movable between three positions along the axis of rotation. In a first position, the slip-engagement member does not engage the first or the second adjustment members. In the second position, the slip/engagement member engages the first adjustment member to rotatably adjust only the first adjustable member about the axis of rotation as the slip/engagement member is rotated about the axis of rotation. In the third position, the slip/engagement member engages the second adjustable member to rotatably adjust only the second adjustable member about the axis of rotation.