Collecting Binoculars
By Mel Lewis

The technology which led to the invention of binoculars was known from the 17th century. There are two main types of telescope: the astronomical version, where the image is inverted; and the Galilean, where the eyepiece lens rights the image, but offers a narrow angle of view. The prism system with its familiar "dog-leg" shape dates to an 1854 invention by the Italian, P.I. Porro. Binoculars are simply a pair of telescopes with a convenient focusing device. A monocular is merely half a binocular. The technology behind field glasses, binoculars without prisms - once the first choice for the WWI military - lives on in opera glasses, where low light ability is vital. A post-World War II innovation was the "roof prism" system, enabling both front and back lenses to be on the same axis, thereby doing away with the dog-leg. Virtually all collectable optical instruments share the hazards of fungus, chipped lenses and "collimation" - prism displacement. Check for collimation by viewing through the wider lens, towards a bright light source. Double images give the game away. Fungus creates a milky blot on the lens. However, dismantling prismatic binoculars for repair or cleaning is best left to an expert: professional overhaul costs from £50 ($80).

Binoculars are usually marked with two numbers: their power and the size of the objective lens in millimetres. Thus a pair marked 8x30 will magnify an image eight times, making something 800 yards distant appear to be only 100 yards off. But it isn't all about power. Other factors, such as field of view ... resolution ... weight ... speed of focusing, all count. With centre focus binoculars, locate a distant but clearly outlined object 300-400 yards away; a chimney or telegraph pole, say. Close the eye against the adjusting eyepiece, usually on the right side. Turn the knurled central screw to focus for the left eye. Finally adjust the right eyepiece, left eye closed, until the image is sharp. You have now compensated for any eyesight imbalance and can use just central focusing.

With traditional binoculars the well focused image is startlingly three-dimensional. This is because the objective lenses are a lot further apart than human eyes, which exaggerates the stereo effect. Some collectors specialise by favourite makers, or country of origin. For example, Ross, Wray, Kershaw, Barr & Stroud (UK); Leitz, Zeiss, Busch, Goertz, Hertel & Ruess (Germany); Bausch & Lomb (USA). In the absence of a maker's name, WWII military glasses are sometimes revealed by the presence of a terrain scale or grid, used to estimate distance. The letters "bch" or "beh" indicate WWII issue Leitz (makers of Leica equipment) military issue binoculars - a find indeed. Last year Hitler's own Carl Zeiss binoculars made £28,125 ($45,000) in a Swindon saleroom. At the other end of the scale, Beshnell Corp, USA, make an amusing miniature Coca-Cola binoculars, 8x21 power, with lens caps made to look like the crown caps on the classic Coke bottle. Price? Just £49.99 ($80) for a surefire future collectable with cachet.