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Binoculars Explained

This guide aims to introduce you to the complex field of binoculars and help you make an informed decision when purchasing for your requirements.
About Binoculars
Binocular Terms
Binocular Properties

Humans have always harboured the desire to see further than is possible when using the naked eye.This was first made possible in the 1600's when the telescope was invented by a Dutch optician. Since then, improvements in lens and body design, and the parallel tube arrangement have contributed to make binoculars what they are today. The modern compact binocular is generally small, yet with stereoscopic vision and depth of field far greater than before.

Types of binoculars
Before describing the various configurations used in binocular design, a few terms should be introduced:

  • Objective lens - this is the large lens at the far end of the binocular
  • Ocular lens - the lens in the eyepiece
  • Prism - a block of glass with triangular section having accurately polished edges, used to reflect light

Prisms are introduced into prismatic binoculars to enable the body to be physically reduced in length compared to that of a telescope. They also ensure that the image is correctly orientated.

Porro Prism
This consists of two right-angled prisms joined so as to reflect the light path 3 times, resulting in a corrected image. The conventional shaped binoculars, (shown right), use Porro prisms.
Example of porrow prism binoculars

Example of alternate design porrow prism binoculars For a more compact design, the objective lenses can be placed closer together, resulting in the 'M' or inverted porro prism style as used in the Monk Explorer (left). The closer spacing of the objective lenses in this configuration tends to reduce the stereoscopic effect when compared to the standard porro prism configuration

Dach (or Roof) Prism

This is technically more complicated than the Porro prism design, with prisms requiring very precise angles and polishing. The light path is reflected 4 times. This design is more compact, but as the light path is longer and the prisms more complicated, the glass and manufacturing need to be of higher quality than the Porro prism design for equal performance.
Roof prism binoculars tend to have straight barrels, such as the Fumoto Cadet, shown right.

roof prism or dach prism binoculars
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Zoom binoculars

Zoom binoculars have part of the ocular lens arrangement moveable so as to adjust the magnification, typically from about 7x to 20x. Unfortunately, although the idea is a good one, zoom binoculars tend not to work very well due to the large number of moving parts. In order to keep the images clear and in collimation (see later for definition), the quality of engineering needs to be very high. For this reason, most zoom binoculars tend to go out of collimation and have a blurred or double image at the higher magnifications. Monk Optics therefore recommends that a fixed magnification is always used to avoid fatigue and uncomfortable viewing.

Specialist binoculars

Although the general design remains the same, binoculars are available for specialist purposes. Waterproof binoculars sometimes incorporating a compass are the usual for marine use, and larger more powerful binoculars are used for long distance observation and astronomy.
This website shows a variety of binoculars for different uses, together with more information and advice about choosing the best one to suit you.
Some of the specialist products available from
Monk Optics.....

About Binoculars
Binocular Terms
Binocular Properties

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