Monday, March 5, 2012

A birder's best friend. A short history of bioculars

Old and New.  An 1890s 12x30
field glass next to a current
Zeiss 8x30.
We have recently filmed a short video guide to understanding the specifications of binoculars (Click here to view), so as a follow up this is a (very) brief history of binoculars with some additional information on types and specifications. 
Binoculars are the one essential piece of equipment for the birdwatcher and are a great boon to most other outdoor activities. The history of these elegantly simple instruments started in 1608 when the Dutch spectacle  maker Hans Lipperhey built a “telescope for both eyes”.
This was essentially two telescopes joined together, producing an inverted image.  Significant advances in the technology beyond the basic Galilean design of two lenses at either end of a tube came later in the Seventeenth Century with the addition of erecting lenses to correct the inverted image. These additional lenses made the glasses much more practical for terrestrial use but increased the length significantly.

Ernst Abbe 1840-1905.
Father of the prismatic binocular.
The Porro prism design.
In 1854 the Italian artillery officer Ignazio Porro developed a telescope that erected the image by using two prisms. The Porro prism design (and other arrangements of prisms) effectively folds the light-path allowing much shorter instruments to be made. When independently reinvented by Ernst Abbe in 1870 (who worked in association with optics company Carl Zeiss in Jena) this much more compact design formed the basis of modern prismatic binoculars. During the Twentieth Century advances in optical understanding and technology included coated lenses to increase light transmission and colour fidelity, nitrogen filling for fog and water proofing and more recently fluorite crystal lenses with their very low dispersion and excellent light transmission.

These days there are essentially two types of binocular widely available: roof prisms and Porro prisms. The majority of modern binoculars are of the roof prism design which has a more slim-line shape and when combined with internal focusing gives good waterproofing and shock resistance.  Schmidt type roof prisms are the most commonly used

The Nikon E II 8x30
Dialyt (left) and Schimidt (right) type roof prisms

Top of the range.
A cut away Zeiss Victory FL 8x42 with
Schmidt type roof prism

Although Porro prisms are mostly at the cheaper end of the market due to there simpler construction, there are still some high-quality examples available such as the Nikon E II and the Swarovski Habicht. Modern top of the range binoculars are ultra-precise and constructed under laboratory conditions using exotic materials, complex and time-consuming process and rigorous testing.

Here are some of the terms associated with binoculars that you might come across and what they mean.

APO. Abbreviation of apochromatic. A completely apochromatic lens system corrects all chromatic aberration (colour fringing).  Leica's APO-Televid scopes also have fluorite lens elements.

BK-7 and BAK-4 prisms. These are two grades of glass, boroscilicate BK-7 (generally in cheaper optics) and barium crown BAK-4 (delivering better sharpness)

ED, HD, HR. Terms used to denote higher-quality glass models, HD standing for High Definition, ED usually standing for Extra-low Dispersion, and HR standing for High Resolution. These terms are not standardised, one companies standard glass can sometimes be as good as another's HD. With Leica and Swarovski HD denotes models with fluorite lenses.

Eye relief. This is the distance that your eye should be from the eyepiece lens to get the optimum image. Spectacle wearers often need a longer eye relief when using binoculars with there glasses on.

Nitrogen filled. Waterproof binoculars are often filled with a dry, inert gas (most commonly nitrogen or argon) to prevent internal fogging.

Phase correction. Coatings applied to the surfaces of roofprisms to reduce dispersion, giving sharper images with better contrast and reduced chromatic aberration (colour fringing).

For any more information please contact us through our website or drop in and see us at Glandford or Cley Marshes.

Cley Spy

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