39 : Porro-prism binocular

1894

Porro-prism binoculars were an improvement on opera glasses, and utilised a small series of mirrors aligned inside the wide body. The same basic design remains in use today, though in the eyes of many birders they have been superseded by more compact roof-prism models.

What birder would be seen – or indeed see anything – without his or her trusty ‘bins’? The binocular is a birding essential, as crucial as a pair of ears and more necessary than even a field guide. And not only do those reliable double handfuls bring birds closer, their invention and evolution could also be said to have driven the development of birding as a hobby, in a mutual marriage of tool and usage.

Binoculars are essentially two identical telescopes aligned physically and through a single focusing system, and designed to be held in both hands. The slightly differing viewpoints of each eye give the impression of depth to the image, a convincing illusion of three dimensions.

Original binoculars – which literally were two small-magnification telescopes joined together – are termed Galilean and have a convex objective lens and a concave eyepiece lens. Such a construction formed the basic design of opera glasses and other earlier-19th century optics.

Perhaps the first binocular revolution was the invention of porro-prism binoculars, but like all revolutions, it didn’t happen overnight. In 1854 Ignazio Porro, an Italian optician, patented his system of twice-reflecting an image in internal mirrors so that it retained its natural orientation to the eye, but it wasn’t commercially developed until 1894, when Carl Zeiss began selling the first recognisably modern binocular.

Porro-prism binoculars are generally small, and have a bright and true image. The design stayed popular through to the late 20th century, but their industry primacy – at least in birding terms – was gradually usurped by roof-prism models. Roof prisms first emerged in the 1870s and were successfully commercially patented in 1905, again by Carl Zeiss. The direct alignment of lenses in roof-prism binoculars means that the barrels could be slimmer and more compact, but lose a little comparative brightness (though this can be compensated for in other ways such as lens coatings in modern models).

The continued use of binoculars for sports and birding, particularly that of roof-prism models, has resulted in many technical innovations during their evolution. Anyone who has ever tried to focus on a dim and distant bird in cold temperatures, drizzle or outright rain will, for example, appreciate the development of waterproofing and fog-proofing. Commercial hermetic sealing of binoculars, which prevents water from getting inside, and gas-filling, to enable fog-proofing, date from the early 1970s. A waterproof binocular will have O-rings at each opening in its structure which aid focusing and act as internal washers, and the entire space inside is filled with gas (often nitrogen) by injection, then completely sealed.

The nitrogen chemically prevents the condensation caused by extreme changes and juxtapositions of temperature. The absence of oxygen negates corrosion on the optics’ internal workings. However, even now, it is impossible to prevent all the actions of water vapour in optics, and users of even the most expensive brands will find that the minute amount of heat and water from their eyes will sometimes fog up their lenses in cold weather. Rubber armour typically completes the sealing of a binocular, preventing internal water access and damage, as well as protecting the outer casing. Most are also tested underwater to see if both the rubber armour and metal housing can withstand pressure of up to 5m for five minutes, avoiding damage and water intrusion.

The optics market is now crowded with budget, mid-price and high-end options, with several major brands competing for the high-spending birder. Innovation remains ongoing, providing ever clearer, truer and brighter images for the field birder, while top-end models are sometimes worn as status symbols to mark the supposed skills of individual birders themselves.

Such precision optics have certainly aided the eyesight and expertise of many a birder. The development of the whole industry means that even entry-level binoculars can often be of a quality that only the best and most expensive could provide just a few short decades ago.