Binoculars

Binoculars are an optical aid used to view distant objects with both eyes, such as scenery, wildlife, or far off boats or ships. They can also be employed in basic astronomy, in lieu of a telescope.

They consist of twin barrels containing a system of lenses one for each eye. Because the distance between the eyes varies from person to person, there must be provision for adjusting the distance between the binocular's eyepieces; this is done by hinged the barrels together.

While most binoculars are simply designed to be purely functional, opera glasses are a kind of binoculars that are usually rather ornate in finish. They are used to improve the patron's view of the action on stage. The fancy form fits in with the formal nature of a performance and the highly decorated interior of most theaters.

Settings

A binocular usually has three settings. One of them is used to change focus the binoculars on objects of different distances. The other two are personal settings which vary from person to person but which, for any person, only need to be set once and do not need to be changed subsequently.

1) A focusing knob is usually provided in the center which focusses both sides at the same time, to focus the binoculars on objects of varying distances.

2) The distance between the eyes (the intraocular distance) varies from person to person. In order to allow the distance between the binocular tubes to be changed, they are hinged together at the center. Often there is a graduated scale at the hinge from which the intraocular distance can be read, usually in millimeters. If a user remembers their intraocular distance, they can quickly bring any pair of binoculars back to setting that is proper for them.

3) A second focussing adjustment is provided, usually on the right eyepiece, usually with a scale and markings in diopters. For many people, this adjustment should be set to the zero mark. Some people will find that because of refractive differences in their eyes, when the left binocular is perfectly focussed, the right binocular will be slightly out of focus. It can be brought into focus by adjusting the right eyepiece focus. Once the right eyepiece focus is set correctly, it will be correct at all distances and does not need to be adjusted again.

Designs

Because they are used to view terrestrial objects, the images in a pair of binoculars need to be erect, i.e. right-side-up. This poses an engineering challenge. There is a kind of telescope, the Galilean telescope, which uses only lenses and provides an erect image. Some instruments use a pair of Galilean telescopes. This design is inexpensive, can be compact, and can produce a sharp image; however, their magnification, field of view, and "exit pupil size" (which determines brightness) are all limited. This design was once popular for opera glasses, but today is found mostly in children's toy binoculars.

Most modern binoculars are prismatic binoculars. The lens systems used in them are similar to the design of a "astronomical" refracting telescope, but a system of erecting prisms is added to turn the image right side up.

In the traditional design, the presence of a prism system is visible in a dogleg appearance of the two barrels, the eyepieces being offset and out of line with the main barrel. However, in recent decades, optics firms have developed very compact prismatic binoculars with prism systems are inline with the barrels with no visible offset.