The first binoculars were really "opera glasses," and these small instruments consisted of two small Galilean telescopes side by side. The combination of a convex objective lens and a concave eyepiece produced a rather limited field of view and a magnification of about three.
By the 1790s the Venetian optician Lorenzo Selva had introduced a central adjustable hinge, enabling the binocular eyepieces to be moved apart or close together. The center-wheel focusing mechanism was introduced about 1830. A Keplerian telescope system of two convex lenses was used for astronomical observations but this had a huge disadvantage for terrestrial use as the image was inverted.
An Italian artillery officer, Paolo Ignazio Pietro Porro (1801-1875), overcame the problem of inversion by placing two prisms in a Z-shaped configuration between each objective lens and eyepiece. This widened the binoculars and separated the objective lenses. It also improved the user's stereoscopic vision and gave a better sensation of depth. The fact that the two optical paths were folded in on themselves also meant that the length of the instrument was less than the focal length of the objective, and this too made the binoculars more convenient. Porro patented his design in 1854, and it was subsequently refined by makers such as Carl Zeiss. The desire to avoid patent infringement eventually led manufacturers to introduce stereo prisms and roof prisms.
Widespread use of binoculars by the military in the two world wars. pushed development of the technology further, with the introduction of antireflection lens coatings, wide-angle eyepieces, and slimline, lightweight versions with nitrogen-filled, rubber-coated, waterproof bodies.
