Monocular
A monocular is a compact refracting telescope used to magnify images of distant objects, typically using an optical prism to ensure an erect image, instead of using relay lenses like most telescopic sights. The volume and weight of a monocular are typically less than half of a pair of binoculars with similar optical properties, making it more portable and also less expensive. This is because binoculars are essentially a pair of monoculars packed together — one for each eye. As a result, monoculars only produce two-dimensional images, while binoculars can use two parallaxed images (each for one eye) to produce binocular vision, which allows stereopsis and depth perception. Monoculars are ideally suited to those application where three-dimensional perception is not needed, or where compactness and low weight are important (e.g. hiking). Monoculars are also sometimes preferred where difficulties occur using both eyes through binoculars due to significant eyesight variation (e.g. strabismus, anisometropia or astigmatism) or unilateral visual impairment (due to amblyopia, cataract or corneal ulceration). Conventional refracting telescopes that use relay lenses has a straight optical path that is relatively long; as a result, monoculars normally use Porro or roof prisms to "fold up" the optical path, which makes it