A pair of binoculars is basically two telescopes positioned side by side. A stereoscopic view can be obtained as both eyes are used to look at the object. This allows subjects to be viewed in depth with considerably less eye strain, than if a single telescope were being used.
The most simple binoculars contain two Galilean telescopes. Each of these telescopes encompass two lenses. These type of binoculars are often referred to as `field glasses' or `opera glasses.' One of the drawbacks of this type is that there is a relatively small field of vision, and they can only magnify the subject up to four times its original size.
A more advanced form utilises the achromatic lenses. These are heavier and longer than the Galilean type. By utilising two right-angled prisms in each telescope, the light can be induced to fold. This causes the distance of the light travelling from the front object glass to the eyepiece to lengthen considerably. This alleviates the need to make the body of the binoculars longer. This method permits the front object lenses to be set further apart and therefore create a greater stereo effect for the viewer. It also allows larger lenses to be used. With a simple telescope design, the image will be seen upside down, however, there are prisms which turn it the right way up, and prevents the need for extra lenses.
To focus the binoculars, a central wheel on the bridge (which is linked to the two eyepiece tubes) can be rotated. By turning the wheel, the tubes can move in or out. To allow for the varying strengths of the individuals eyes, a twist focusing thread on the right hand eyepiece is provided. The two halves of the binoculars are hinged, so that the user can adjust the binoculars to the required distance needed for their eyes.
The effectiveness of a pair of binoculars is determined by two figures. The most commonly known specifications are 8 X 30, 7 X 50 and 10 X 50. Using 8 X 30 as an example, the 8 signifies how many times the object being viewed will be magnified and the 30 indicates the diameter in millimeters of each object glass and therefore the viewing area or `field of vision.'
Some of the most advanced binoculars used for military purposes now allow the operator to see in the dark, by the use of special infra red components.
