The Unique Qualities of Binoculars
Binocular operation begins not with the binoculars, but with human eyes. Human eyes have binocular vision, which works by combining the images produced by the slightly different perspectives of each eye to produce a three-dimensional image with depth perception. Telescopes are monocular, with only one eyepiece, and therefore lack depth perception. This is irrelevant in astronomy, but the difference can be very important for terrestrial observation.
These binoculars are essentially a pair of Galilean telescopes mounted side by side. Galilean optics work by using a convex objective lens, to gather and concentrate light to magnify images, and a concave eyepiece. They are very simple, and cannot achieve very high magnification. Today, this model is found mostly in opera glasses and toy binoculars.
Prism binoculars use Keplerian optics, in which both the objective and eyepiece lenses are convex. They can achieve much greater magnification than the Galilean model. The problem with Keplerian optics is that the image is inverted, a serious defect for terrestrial observation. Prism binoculars correct the problem and create an upright image through use of a prism, placed between the objective and eyepiece lenses.
Many binoculars have functional options. A common additional feature is "independent focus," a control to adjust the focus of each eyepiece independently. This is common in spotting and military binoculars. Spotting and military binoculars also usually have anti-reflective coating on the objective lenses. Some have internal gyroscopes to reduce image destabilization caused by vibrations, which can be a problem with large binoculars that are mounted to a vehicle or boat.