In 1823, a new optical instrument began to appear in French opera houses that allowed patrons in the distant (and less expensive) seats to view the opera as if they were in the front row. Called opera glasses, the device combined telescope lenses with stereoscopic prisms to provide a magnified, three-dimensional view. After many years (but relatively few modifications), opera glasses have evolved into the binocular.
In their simplest form, binoculars are a pair of small refracting telescope lenses, one for each eye. The brain assembles the two views, one from each lens, into a single picture. Because each eye sees its own view, the final image has depth; this is not so with conventional telescopes, which possess only one eyepiece and, therefore, a two-dimensional image.
While some simple binoculars can be found, most quality binoculars possess a more intricate design. In more complex binoculars, there is a system of prisms between the large front lens, called the objective lens, and the smaller eyepiece. These prisms serve two important functions. First, they bend the light so that the final image is both upright and nonreversed. In a common telescopic view, the image is both reversed and upside-down. Second, the bending produced by the prisms lengthens the overall light path, which allows for much greater magnification while staying within the binocular's short tube. Without prisms, average-powered binoculars would need to be more than one foot (0.305 m) in length.
In order to enhance the stereoscopic, or three-dimensional, effect, the binocular's two objective lenses are placed further apart than the viewer's eyes. When the two views are then assembled by the brain, a greater impression of depth and clarity results.
Many different factors influence the quality of a binocular. For the majority of users, the most important of these is magnifying power. Binocular magnification usually ranges from six to twenty times—that is, the object appears six to twenty times larger in the binoculars than it would with the unaided eye. Magnification is usually expressed as "X," whereby six times magnification would be written 6 X.
Another factor governing a binocular's quality is the size of the two object lenses, called the aperture. Larger aperture sizes are valued, because they collect a greater amount of light. This is crucial, because an image becomes fainter as the magnification increases. Thus, high magnifications are usually coupled with wide apertures. While object lenses of 30-80 mm (1-3 in) are common, apertures as large as 150 mm (6 in) have been designed; these, however, are used chiefly in military reconnaissance. Binocular makers generally express the quality of their instruments in terms of both magnification and aperture size. A common rating is 7 × 50, which describes a magnification of seven power and an aperture of 50 mm (2 in).
Professional users of binoculars, such as astronomers and military personnel, also consider the size of the light beam that exits the eyepiece, called the exit pupil. The closer this light beam is to the width of the viewer's own pupil, the more efficient the binocular. This is a tricky factor because the size of a human pupil varies: in bright light (such as daylight) the pupil is only 0.078 in (2 mm) across, while in dim light (such as moonlight) it opens to almost 0.273 in (7 mm). Thus, binoculars with a small exit pupil are best for daytime use, while those with a wider exit pupil are essential for nighttime observing.
Yet another factor affecting the quality of a binocular is the straightness of its beams. In a perfectly adjusted instrument, the two beams entering each eye will be parallel. If these beams are offset even slightly, a doubled image will be produced. Such a poorly adjusted view is uncomfortable and bad for the eyes. In order to fix the image, the binoculars should be collimated so that the beams are parallel.
Although they are inferior to telescopes in magnification, binoculars are often better devices for viewing the heavens. Because of their wider object lenses, binoculars can collect more light than telescopes; this makes objects such as distant stars or planetary satellites appear much brighter than they would in many telescopes. Even household binoculars are sufficient for viewing the Moon and the visible planets, and they are usually much less expensive than a telescope.