Why roof-prism binoculars must be aligned 300 times more precisely than Porro ones
The two numbers stamped on binoculars, like 10×50, mean magnification and the width of the front lenses in millimetres. Hidden inside is a harder problem: flipping the upside-down image. Roof prisms that do it in a slim, straight tube must be accurate to 2 arcseconds, while bulkier Porro prisms tolerate about 10 arcminutes.
Binoculars are a matched pair of telescopes. Because each eye receives a slightly different view, the brain fuses them into a sense of depth, which a single telescope cannot provide. People tried pairing telescopes almost as soon as the instrument was invented in the seventeenth century. Early models used the Galilean arrangement, a convex front lens with a concave eyepiece, which gives an upright image but a narrow field and low power; opera glasses and jewellers' loupes still use it.
The Keplerian design, with a positive eyepiece, allows sharper images and higher magnification but turns everything upside down. Nineteenth-century twin telescopes fixed this with extra relay lenses, making them long and difficult to build. Ignazio Porro patented a prism system for righting the image in 1854. After Otto Schott produced improved crown glass in 1888, Ernst Abbe and Carl Zeiss launched modern Porro binoculars in 1894, with two prisms in a Z-shape that fold the light path, shorten the body and spread the front lenses apart for stronger depth.
Roof prisms may date to a design by Achille Daubresse in the 1870s, and Moritz Hensoldt began selling pentaprism roof binoculars in 1897. Most now use the Schmidt–Pechan prism of 1899 or the Abbe–Koenig prism patented by Zeiss in 1905. Their lenses line up with the eyepieces, so the instruments are narrower and lighter, and they have been widely used since the later twentieth century. Schmidt–Pechan prisms rely on mirrored surfaces and so pass slightly less light, yet by the early 2020s they had become the most common design.
Field of view shrinks as magnification rises, and a larger objective gathers more light and resolves finer detail. In top-quality binoculars, better coatings and glass have made differences between prism types practically irrelevant.
Source: Binoculars
