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 right way up.Ĭross-section of a relay lens aprismatic binocular design These are typically mounted on an eyeglass frame or custom-fit onto eyeglasses.Īn improved image and higher magnification is 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. Optical designs Galilean Īlmost 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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