page_title: Binoculars
section_title: Porro prism binoculars
hierarchical_section_title: Binoculars / Optical designs / Binoculars with Keplerian optics / Prism binoculars / Porro prism binoculars
caption_attribution_description: Combination of porro prisms used in binoculars
context_page_description: Binoculars or field glasses are two telescopes mounted side-by-side and aligned to point in the same direction, allowing the viewer to use both eyes when viewing distant objects. Most are sized to be held using both hands, although sizes vary widely from opera glasses to large pedestal mounted military models.
  Unlike a telescope, binoculars give users a three-dimensional image: for nearer objects the two views, presented to each of the viewer's eyes from slightly different viewpoints, produce a merged view with an impression of depth.
context_section_description: Porro prism binoculars are named after Italian optician Ignazio Porro, who patented this image erecting system in 1854. This system was later refined by other binocular makers, notably the Carl Zeiss company in the 1890s. Binoculars of this type use a pair of Porro prisms in a Z-shaped configuration to erect the image. This results in binoculars that are wide, with objective lenses that are well separated and offset from the eyepieces, giving a better sensation of depth. Porro prism designs have the added benefit of folding the optical path so that the physical length of the binoculars is less than the focal length of the objective. Porro prism binoculars were made in such a way to erect an image in a small space, thus binoculars using prisms started in this way.
caption: Double Porro prism design
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