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Binoculars & their Terms

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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: June/23/2004 at 11:42
Stephanie View Drop Down
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Joined: February/13/2004
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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, such as the Monk Argonaut 7x50 (shown right), use Porro prisms.
porrow prism example

Dach (or Roof) Prism
roof prism example

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.

Focussing and diopter adjustments

As there is often a difference in focus between the left and right eye, a diopter adjustment is necessary. On most centre-focus binoculars the left eye is first focussed using the central wheel, after which the right eye can be balanced using the diopter adjustment on the right eyepiece (refer to figure 14). N.B. It is a very common misconception that the right eye dioptre adjustment should be made before the centre-focus whereas it should in fact be the other way round! Once the dioptre adjustment has been set, the main central wheel will focus both eyes equally.
Binoculars having individual eye focussing allow the diopter of both eyepieces to be set individually. However a change of object distance requires t
he adjustment of both eyepieces unless the depth of field is particularly good


The first figure describes the magnification: 7x, or 10x etc. This means that the image seen using the binoculars will be 7x and 10x bigger than when viewed with the naked eye. For example a bridge 1000 metres away would appear to be 100 metres away when viewed with 10x binoculars.
This is illustrated in the two diagrams below:

binocular view - 7X binocular view - 10X

Another important quality of the binocular is the Lens Coating used. Coatings help to reduce reflected light and increase the percentage of light reaching the eye. The effects of coatings can be seen in the diagrams (right).

It can be seen that the reflected light is dramatically reduced on the coated lens. A single uncoated objective lens will transmit about 95% of all incident light, while a coated lens will transmit about 99% (depending on coating type and quality). Therefore in a binocular with 10 surfaces (lenses and prisms) no coatings will result in an overall light transmission of about 65% whilst good coatings can improve this up to 95% resulting in a much brighter image.

lens coatings Lens has no coating
Coated Lens lens coatings
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