Of all the optical instruments, apart from cameras, the most popular is the binocular telescope. It allows people to watch sports days and concerts more closely, adding a lot of fun. In addition, the binocular telescope provides a sense of depth that monocular telescopes cannot match. The most popular binocular telescopes use convex lenses. Since the convex lens reverses the image up and down and left and right, a set of prisms is required to correct the inverted image. Light passes through these prisms and travels from the objective lens to the eyepiece, where it is reflected four times. In this way, the light travels a long way over short distances, so the barrel of a binocular telescope can be much shorter than that of a monocular telescope.