A diopter adjustment mechanism comprises a scope tube with a first female thread and a second female thread. The first female thread has a first pitch and is disposed on the scope tube forming a spiral thread and lands between the spiral threads. The second female thread has a second pitch and is disposed on the lands of the first female thread. A jam nut has a male thread corresponding to and engaging the second female thread. The pitch of the first female thread is greater than the pitch of the second female thread.