Binoculars are a pair of identical telescopes that are mounted next to each other so that they point in the same direction. This allows the user/viewer to use both his eyes to view objects at a distance. Binoculars provide the viewer with a three dimensional image of the object. There is no need to close one eye as is the case with regular telescopes. Binoculars are also often referred to as field glasses.
They have different means of adjusting the distances between the inner prisms and mirrors and lenses in order to vary the distance being viewed. Depending upon the type used, they may have either a central adjustment wheel or each lens can be adjusted independently. These binoculars may also have different systems to take care of image stabilization by providing for internal mechanisms to soften the effect of movement.
While using a pair of binoculars, there are a variety of terms which are used. Let us look at these terms.
This is the ratio of the focal length of the eyepiece divided into the focal length of the objective. This is the magnifying power of the lenses. For example, if a pair of binoculars has a magnification of 5, this means that the image produced is as if the object being viewed were 5 times closer to the viewer. Usually, the larger the magnification, the smaller is the field of view.
This is the diameter of the objective lens. This determines the amount of light which can be gathered to form an image. Usually, this is expressed in millimeters.
Field of View
This is the width of the view which will be seen at a distance of 1000 meters (or yards) or it is the angular value of how many degrees can be viewed from the lens.
The light gathered from the objective lens is directed into the exit pupil. The diameter of the exit pupil is equal to the diameter of the objective lens divided by the magnifying power of the lens. Ideally, the diameter of the exit pupil should be equal to the diameter of a fully dilated iris of the human eye (or about 7 millimeters). A larger exit pupil makes for easier viewing due to ease in eye alignment.
This is the distance from the rear eyepiece of the lens to where the image is formed. This determines how far the viewer must hold his eyes from the rear eyepiece in order to see the image properly. The longer the focal length of the eyepiece, the more is the eye relief.
Binoculars are differentiated by the type of lenses and prisms used in them to produce the effect of bringing the image closer in view.
These are typical binoculars found everywhere. They use a pair of convex and concave mirrors to produce the effect of bringing the image closer to the viewers' eyes. However, these binoculars have a narrow field of view and are not capable of creating a high degree of magnification. They find their uses in the opera glasses in theaters and simple binoculars which are used as toys.
Porro Prism Binoculars
A system of creating the binocular-effect was presented in 1854 by Ignazio Porro using a double prism in a Z-shaped configuration. The resulting binoculars are wide and the objective lenses are quite separated from each other. However, the length of these binoculars is less because the design of the Porro Prisms is such that they fold the optical path.
Roof Prism Binoculars
These binoculars use the Abbe-Koenig prism, named after Ernst Karl Abbe and Albert Koenig who devised it. This was later patented in 1905 by Carl Zeiss. These binoculars have objective lenses which are in line with the eyepieces.
Binoculars have found their uses in various places. People even use them for hobbies like birdwatching. Binoculars are also particularly used by the military for their operations. Some amateur astronomers have also been known to use them to view celestial bodies in the sky.