Binoculars work like a pair of matching telescopes, except they provide a magnified stereoscopic view of faraway objects. Their main advantages over a single lens monocular (telescope) is that they provide depth perception, they’re more comfortable as they allow viewing with both eyes and the visual acuity is better because the brain has two sets of visual data to process.
The basic job of binoculars is to prescribe a folded path for the light entering the objective lens through to the eyepiece, and ultimately the operator’s eye, so that the appropriate magnification can be achieved without increasing the overall length of the binoculars.
Each of the lenses has a pair of reflecting prisms inside that re-invert (ie flip and reorient) the image received by the lens.
The most common type of binoculars incorporate Porro prisms (named after their Italian inventor, Ignazio Porro). However roof, or Dach, prisms are being used in some newer models that allow for a straight profile, with the eyepiece in line with the objective lens (as found in more compact binoculars). They’re more comfortable and easier to manage than Porro-prism types, but need more careful aligning and lose more light, plus are generally more pricey than Porro-prism instruments.
Binocular optics are generally described by two figures. For example, the 10 in a 10×42 pair of binoculars refers to the magnification, while the 42 refers to the diameter of the objective lens in millimeters.
Binocular tech under the microscope
Modern binoculars employ a number of optical technologies in order to improve visual performance. The most common are the different optical coatings: a phase-correction coating on the surface of a roof prism, for instance, is used to compensate for the reduction in contrast and resolution that compact binoculars are known for. Anti-reflective coatings on every optical surface in certain binoculars can improve image contrast, while mirror coatings of aluminium or silver are used in some models of roof-prism binoculars to reduce the amount of light that is lost.
Binoculars which have a high degree of magnification may include some form of image stabilization too. This is used because even the slightest tremor in the hands can significantly distort the image. These are often powered mechanisms driven by gyroscopes that dampen the effects of small, unintentional movements, although image-stabilization technology tends to increase both the weight and price-tag of the binoculars.