The Italian inventor of optical instruments Ignazio Porro worked during the 1860s with Hofmann in Paris to produce monoculars using the same prism configuration used in modern Porro prism binoculars. Optical prisms added to the design enabled the display of the image the right way up without needing as many lenses, and decreasing the overall length of the instrument, typically using Porro prism or roof prism systems. The Keplerian "twin telescopes" binoculars were optically and mechanically hard to manufacture, but it took until the 1890s to supersede them with better prism-based technology. Such binoculars were popular in the 1800s (for example, G.& S. The binoculars with erecting lenses had a serious disadvantage: they are too long. These lenses are used to erect the image. In aprismatic binoculars with Keplerian optics (which were sometimes called "twin telescopes"), each tube has one or two additional lenses ( relay lens) between the objective and the eyepiece. Since the Keplerian configuration produces an inverted image, different methods are used to turn the image the right way up. These are typically mounted on an eyeglass frame or custom-fit onto eyeglasses.Īn improved image and higher magnification are achieved in binoculars employing Keplerian optics, where the image formed by the objective lens is viewed through a positive eyepiece lens (ocular). They also have large exit pupils, making centering less critical, and the narrow field of view works well in those applications. The Galilean design is also used in low magnification binocular surgical and jewelers' loupes because they can be very short and produce an upright image without extra or unusual erecting optics, reducing expense and overall weight. This type of construction is still used in very cheap models and in opera glasses or theater glasses. The Galilean design has the advantage of presenting an erect image but has a narrow field of view and is not capable of very high magnification.
Most early binoculars used Galilean optics that is, they used a convex objective and a concave eyepiece lens. Almost from the invention of the telescope in the 17th century the advantages of mounting two of them side by side for binocular vision seems to have been explored.
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