Binoculars
Binoculars or field glasses are two refracting telescopes mounted side-by-side and aligned to point in the same direction, allowing the viewer to use both eyes when viewing distant objects. Most binoculars are sized to be held using both hands, although sizes vary widely from opera glasses to large pedestal-mounted military models.
Unlike a telescope, binoculars give users a three-dimensional image: each eyepiece presents a slightly different image to each of the viewer's eyes and the parallax allows the visual cortex to generate an impression of depth.
Optical design evolution
Galilean
Almost from the invention of the telescope in the 17th century the advantages of mounting two of them side by side for binocular vision seems to have been explored. Most early binoculars used Galilean optics; that is, they used a convex objective and a concave eyepiece lens. The Galilean design has the advantage of presenting an erect image but has a narrow field of view and is not capable of very high magnification. This type of construction is still used in very cheap models and in opera glasses or theater glasses. The Galilean design is also used in low magnification binocular surgical and jewelers' loupes because they can be very short and produce an upright image without extra or unusual erecting optics, reducing expense and overall weight. They also have large exit pupils, making centering less critical, and the narrow field of view works well in those applications. These are typically mounted on an eyeglass frame or custom-fit onto eyeglasses.Keplerian
An improved image and higher magnification are achieved in binoculars employing Keplerian optics, where the image formed by the objective lens is viewed through a positive eyepiece lens.Since the Keplerian configuration produces an inverted image, different methods are used to turn the image the right way up.
Erecting lenses
In aprismatic binoculars with Keplerian optics, each tube has one or two additional lenses between the objective and the eyepiece. These lenses are used to erect the image. The binoculars with erecting lenses had a serious disadvantage: they are too long. Such binoculars were popular in the 1800s. The Keplerian "twin telescopes" binoculars were optically and mechanically hard to manufacture, but it took until the 1890s to supersede them with better prism-based technology.Prism
Optical prisms added to the design enabled the display of the image the right way up without needing as many lenses, and decreasing the overall length of the instrument, typically using Porro prism or roof prism systems. The Italian inventor of optical instruments Ignazio Porro worked during the 1860s with Hofmann in Paris to produce monoculars using the same prism configuration used in modern Porro prism binoculars. At the 1873 Vienna Trade Fair German optical designer and scientist Ernst Abbe displayed a prism telescope with two cemented Porro prisms. The optical solutions of Porro and Abbe were theoretically sound, but the employed prism systems failed in practice primarily due to insufficient glass quality.Porro
Porro prism binoculars are named after Ignazio Porro, who patented this image erecting system in 1854. The later refinement by Ernst Abbe and his cooperation with glass scientist Otto Schott, who managed to produce a better type of Crown glass in 1888, and instrument maker Carl Zeiss resulted in 1894 in the commercial introduction of improved 'modern' Porro prism binoculars by the Carl Zeiss company. Binoculars of this type use a pair of Porro prisms in a Z-shaped configuration to erect the image. This results in wide binoculars, with objective lenses that are well separated and offset from the eyepieces, giving a better sensation of depth. Porro prism designs have the added benefit of folding the optical path so that the physical length of the binoculars is less than the focal length of the objective. Porro prism binoculars were made in such a way to erect an image in a relatively small space, thus binoculars using prisms started in this way.Porro prisms require typically within 10 arcminutes tolerances for alignment of their optical elements at the factory. Sometimes Porro prisms binoculars need their prisms set to be re-aligned to bring them into collimation. Good-quality Porro prism design binoculars often feature about deep grooves or notches ground across the width of the hypotenuse face center of the prisms, to eliminate image quality reducing abaxial non-image-forming reflections. Porro prism binoculars can offer good optical performance with relatively little manufacturing effort and as human eyes are ergonomically limited by their [|interpupillary distance] the offset and separation of big diameter objective lenses and the eyepieces becomes a practical advantage in a stereoscopic optical product.
In the early 2020s, the commercial market share of Porro prism-type binoculars had become the second most numerous compared to other prism-type optical designs.
There are alternative Porro prism-based systems available that find application in binoculars on a small scale, like the Perger prism that offers a significantly reduced axial offset compared to traditional Porro prism designs.
Roof
Roof prism binoculars may have appeared as early as the 1870s in a design by Achille Victor Emile Daubresse. In 1897 Moritz Hensoldt began marketing pentaprism based roof prism binoculars.Most roof prism binoculars use either the Schmidt–Pechan prism or the Abbe–Koenig prism designs to erect the image and fold the optical path. They have objective lenses that are approximately in a line with the eyepieces.
Binoculars with roof prisms have been in use to a large extent since the second half of the 20th century. Roof prism designs result in objective lenses that are almost or totally in line with the eyepieces, creating an instrument that is narrower and more compact than Porro prisms and lighter. There is also a difference in image brightness. Porro prism and Abbe–Koenig roof-prism binoculars will inherently produce a brighter image than Schmidt–Pechan roof prism binoculars of the same magnification, objective size, and optical quality, because the Schmidt-Pechan roof-prism design employs mirror-coated surfaces that reduce light transmission.
In roof prism designs, optically relevant prism angles must be correct within 2 arcseconds to avoid seeing an obstructive double image. Maintaining such tight production tolerances for the alignment of their optical elements by laser or interference at an affordable price point is challenging. To avoid the need for later re-collimation, the prisms are generally aligned at the factory and then permanently fixed to a metal plate. These complicating production requirements make high-quality roof prism binoculars more costly to produce than Porro prism binoculars of equivalent optical quality and until phase correction coatings were invented in 1988 Porro prism binoculars optically offered superior resolution and contrast to non-phase corrected roof prism binoculars.
In the early 2020s, the commercial offering of Schmidt-Pechan designs exceeds the Abbe-Koenig design offerings and had become the dominant optical design compared to other prism-type designs.
Alternative roof prism-based designs like the Uppendahl prism system composed of three prisms cemented together were and are commercially offered on a small scale.
Optical systems and their practical effect on binoculars housing shapes
The optical system of modern binoculars consists of three main optical assemblies:- Objective lens assembly. This is the lens assembly at the front of the binoculars. It gathers light from the object and forms an image at the image plane.
- Image orientation correction assembly. This is usually a prism assembly that shortens the optical path. Without this, the image would be inverted and laterally reversed, which is inconvenient for the user.
- Eyepiece lens assembly. This is the lens assembly near the user's eyes. Its function is to magnify the image.