The first binoculars were invented in during the Renaissance and based on the same design as Galileo's telescope. Modern binoculars use lenses, prisms and coatings to magnify very distant objects and provide special enhancements, such as adjustable focus and glare protection.
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Early Binocular Lenses
The earliest binoculars employed Galilean optics, a simple method of optical magnification using two glass lenses; one concave and one convex. The concave lens curves inward and is used as an objective lens, the outer lens that captures the initial image. The convex lens curves outward and is the inner eyepiece lens. The two lenses work together to produce a magnified image (although not highly magnified and with a narrow field of view).
Porro Prism Binocular Lenses
Porro prism binoculars incorporate technology developed by Johannes Kepler and Ignazio Porro. Keplerian optics uses two convex lenses instead of one convex and one concave lens. This allows for a greater field of view at a higher magnification, but produces an upside-down image. To correct this problem, Porro invented a prism that corrects the image. Two Porro prisms are placed inside of the binoculars in a Z-shaped arrangement to invert the image.
Roof Prism Binocular Lenses
Roof prism binoculars also use two convex lenses, but use roof prisms. A roof prism has two faces turned at a 90-degree angle. Two main types of roof prisms are used in binoculars; Abbe--Koenig roof prism and Schmidt--Pechan roof prism. These two roof prisms have slightly different shapes, but both invert an upside-down image in binoculars.
Binocular lenses also often have special lens coatings that enhance viewing. The lenses are made from glass or high-impact plastics. These materials are then coated with anti-reflective coatings, such as magnesium fluoride, that block out glare from the sun's rays or reflective materials. Lenses are also coated with dielectric and mirror coatings that also enhance viewing and produce a realistic image.
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