An optical assembly includes a barrel including a slot, a lens installed inside the barrel for magnifying an image, a magnification-adjusting mechanism installed outside and around the barrel including an adjusting loop installed outside the barrel in a rotatable manner including a groove on a side facing the barrel, and a roller installed between the adjusting loop and the barrel. The optical machine further includes a rod for adjusting the magnification of the lens. One end of the rod is installed inside the slot of the barrel in a slidable manner, and another end of the rod is installed inside the groove of the adjusting loop in a slidable manner.