# binoculars

Illustration of an early Dutch monocular (1624)

A pair of binoculars , a portable telescope . It is mainly available on the market in a binocular version, which makes it possible to observe objects with both eyes via separate beam paths. The binoculars are offered on the one hand in simple lens construction as theater glasses (opera glasses, Galilei telescope ), on the other hand as prism binoculars ( Kepler telescope ), which are also known colloquially as binoculars . One speaks of monocular in one-eyed construction .

The transition to telescopes is formed by the somewhat longer spotting scopes, which are mainly used with a tripod for reasons of weight and construction . With an extendable tube latter also will extract telescopes called.

Typical magnifications are 3 times for opera glasses, 6 to 10 times for binoculars and 15 to 50 times for spotting scopes with zoom eyepieces.

Porro prisms - binoculars, cut open.
Objective , porro prisms and eyepiece

## History and names

Binoculars Dialyt ( M. Hensoldt & Sons in Wetzlar , 1905)
Theater glass (early 20th century)
Polish-French night vision monocular MTN-1 on a combat helmet (2009)

In the 19th century glasses, with the help of which one could see more clearly at a distance than without aids, were called piercing glasses (often designed as monocles ) and the already existing theater glasses were briefly called engravers . A more elegant term was "Lorgnette", which originally applied to glasses to be held in the hand. The term binoculars arose from this for the glasses used mainly by the military in the field .

Theater glasses have been increasingly manufactured since the beginning of the 18th century, when achromatic lenses were available. Johann Friedrich Voigtländer received an imperial privilege to manufacture them in Vienna in 1823. On July 9, 1893, the Carl Zeiss company in Jena applied for a patent for binocular binoculars with prisms similar to the already known and monocularly used Porro prisms . M. Hensoldt & Söhne in Wetzlar built a prism telescope with roof prisms and the name Dialyt (with two objective lenses each ) and also parallel offset optical axes from 1905 onwards. On April 14, 1905, this company registered a patent for a "straight-sighted" Prism binoculars with roof prisms.

The word binoculars is used by many manufacturers as a generic term to denote different types of optical devices with binocular and monocular construction. In specialist shops, however, the term is often limited to binocular prism binoculars, without including theater glasses, spotting scopes, monoculars (small prism telescopes ) and other telescopes . Developers and patenters (e.g. Moritz Hensoldt at the end of the 19th century) still spoke of the prism binoculars ; later the term binocles came into use: In an advertisement by Voigtländer & Sohn from 1907, “prism binocles - for sport, travel, hunting, theater and military service” was advertised.

In English there is no linguistic equivalent that encompasses the same heterogeneous product group as the German term "binoculars". There a distinction is usually made between binoculars or binocular telescopes (binoculars), monoculars or monocular telescopes (monoculars) and spotting scopes (spotting scopes). In international advertising language, the term Sport Optics (also: Sports Optics ) is sometimes used for the product area.

## Types of binocular binoculars

The two main types of binocular binoculars are the optically simple theater glasses and the more complex prism binoculars (binoculars, large binoculars and double spotting scopes).

The traditionally built theater glasses, formerly also called opera glasses, theater binoculars or Galilean binoculars , are light and small binocular binoculars based on the Galileo telescope principle. They contain only one convex objective lens and one concave eyepiece lens per beam path, which directly generate an upright image. Erecting prisms are therefore not necessary. Theater glasses allow only a low magnification (between 1 12 and 5, often made in the versions 2.5 × 18 to 3 × 28), but this is appropriate for observing what is happening on a theater stage. The simple optical construction allows only a small field of view and the image quality is lower than in the compact binoculars with erecting prisms. For this reason, prism theater glasses with the appropriate, relatively small magnification were offered soon after the inversion principle was invented.

Prism binoculars work according to the Kepler telescope principle and contain an inverting prism between the eyepiece and lens, which on the one hand rotates back the image rotated by 180 ° and on the other hand enables a compact design with high-quality imaging.

In terms of size, a simplified distinction is made between compact or pocket binoculars (which are also offered as theater and museum glasses, weigh up to about 300 g and can often be folded into a smaller volume when not in use), universal glasses (binoculars in the narrower sense, between about 400 and 1200 g) and large binoculars with lens openings from around 66 mm, which are more likely to be used on tripods. The names vary a bit.

## Construction methods and product variety of prism binoculars

### Inverting prism variants

Two different erecting prisms are used:

• For a long time, porro prisms dominated binoculars .
• Since the second half of the 20th century, binoculars with straight roof prisms (for example: Schmidt-Pechan prisms ) have been in use on a large scale . Because of the straightness , the latter are particularly compact. Their production is more complex in order to be sufficiently precise. They therefore lagged behind Porro prism binoculars for a long time and could not gain broad acceptance. Even today one observes considerable differences in quality between the manufacturers and the price categories with this type of construction, because maintaining close manufacturing tolerances is difficult and costly. Porro prism binoculars, on the other hand, have a greater production tolerance and require less expensive remuneration for good imaging performance. With a limited budget, you usually get a better optical system in Porro construction than in roof prism construction. Overall, both systems have various specific advantages and disadvantages in terms of manufacturing and ease of use.

Depending on the brand and model, the roof prism glasses appear either rather short and compact or rather elongated and narrow. The reason can lie in different roof prism systems or in different focal lengths of the lenses: Long lens and eyepiece focal lengths make it easier to correct image field curvature and edge blurring and thus often enable better optical characteristics (large field of view, minor aberrations). Short models are lighter and easier to handle.

Larger prism binoculars that have to be permanently installed or placed on a tripod for a steady image are called large field glasses. They are offered with lenses between around 66 and 100 mm. As a result of the large lenses, they can practically only be created using Porro prismatic construction. In terms of size, they lead to double spotting scopes , which often have high-quality apochromatic optics, which would be too unwieldy for normal binoculars. Particularly large, complex and expensive devices are sometimes used by the military and border security.

### Construction methods and quality aspects

The two halves of binocular prism binoculars can be swiveled around the central axis in order to be able to adjust the beam paths to the interpupillary distance (more precisely: pupil distance) of the respective user. There is usually an adjustability of about 56 to about 74 mm (distance between the eyepieces ), only with a few glasses up to about 78 mm. Foldable compact binoculars allow settings from around 32 mm. With an optimal setting and a view into the distance, the observer recognizes an image circle (and not two overlapping like a horizontal 8, as is often shown schematically, for example in movies).

For relaxed stereoscopic vision, it is important that the two beam paths are focused synchronously . In the event of unsynchronized operation or misalignment of the optical parts, which can occur in particular with cheap products, the observer's brain tries to bring the different images together, which is tiring and can cause headaches. In Porro prism binoculars, the eyepieces are linked by a bridge for synchronization and can be moved back and forth by turning a focusing wheel. Roof prism binoculars usually have an inner focusing with a synchronous shift of a lens group in the two beam paths.

The adjustment to the often slightly different refractive power of the left and right eye requires a diopter compensation , which can be done by turning one of the eyepieces (rarely both), sometimes, especially with higher quality binoculars, by adjusting a center drive. With compact binoculars that are very small and light, the diopter adjustment is achieved by separately adjusting the two eyepieces; Some marine glasses also use this functional principle for better water resistance.

The often rubberized connection between the eyepiece and eye should keep disturbing side light as far away as possible. This works most effectively with soft black eyecups that are elongated on the outside and cling to the temples. However, these can only be used without glasses. Eyeglass wearers have to fold them down or (depending on the make) replace them with other eyecups. Many manufacturers have developed eyecups that can be easily twisted out, which can be adjusted quickly and variably, but do not protect against side light as effectively.

Whether or not spectacle wearers can enjoy similarly good visual comfort as non-spectacle wearers depends heavily on the eye-eyepiece distance , which can be adjusted within a certain range by moving or (more comfortably) turning the eyecups. The exit pupil of the binoculars must generally be axially and parallel (“centric”) as well as at a certain distance from the entrance pupil of the eye, ie from the opening of the iris behind the cornea. Depending on the thickness of the glasses, this distance should be about 14-20 mm and it should be lockable. If the eye is too far away, the viewer sees a cropped image; if it is too close to the eyepiece, black shading (so-called kidney beans ) can appear if the view is not exactly centered , which results in uncomfortable vision. Eyepiece constructions that keep this distance are called "eyeglass wearers". Older and inexpensive binoculars are often not suitable for people who wear glasses.

In most cases, viewing pleasure without glasses is higher than with glasses because of the better shielding of the beam path from side light, provided that the respective ametropia can be appropriately corrected using binoculars; Greater corneal curvature ( astigmatism ) cannot be corrected on the eyepiece . However, taking off and putting on glasses is often a hindrance and too slow for many situations (bird watching, hunting, sports events). In addition, possible differences between the two eyes can usually only be set up to about ± 3–5 dioptres and that full focusability of the binoculars is usually only possible up to about 3–8 dioptres for both eyes. In the case of severe myopia, the setting to infinity is no longer possible, while on the other hand the close-up focus is improved. Some manufacturers offer to reduce the so-called overtravel (the range that can be adjusted beyond infinity for people with normal vision) by reworking in the factory at the expense of the close focus.

Important quality criteria are also comfortable handling (grip, focus, accessibility of the focusing wheel). Naturally, people with large and small hands have different requirements here. In addition, feel and functionality are important under different weather conditions (including cold and rain), which includes a mechanically perfect, neither too easy nor too difficult adjustment of the interpupillary distance, the focus and the diopter adjustment. Further important criteria are good suitability for use with and without glasses, tolerance to temperature extremes and moisture, good protection against impact and against scratches on the lenses, and low weight. Aluminum , magnesium alloys or polycarbonates are used to save weight and at the same time make the housing stable . Lens threads are useful for special applications, e.g. for attaching polarization filters for observations near the surface of water (water sports, water birds). Animal watchers and astronomers also appreciate a thread that allows it to be screwed onto a tripod. Additional equipment such as compass and reticle are available for nautical purposes, rangefinders and aiming aids based on laser beams are available for hunters. Some manufacturers also offer image stabilized binoculars, which can compensate for trembling hands. Devices with other electronic additional functions (altimeter, temperature information and time function; Minox BD series ), with an integrated digital camera (Bushnell Imageview) or with data and image transmission between several observers (Leica Geovid Lux) are niche products or devices for special applications. The various types of night vision devices that are not dealt with in more detail here are also designed for special applications (game observation, hunting, police, military, security service) .

### Digital binoculars

In digital binoculars, two sensors behind conventional optics convert the optical image into digital electrical signals. After image processing, these are displayed on two electronic viewfinders . As with digital photography, there is no longer any direct optical connection between the object being viewed and the eye. Electrical energy, which is usually supplied by accumulators, is required for representation. The image processing in connection with the reproduction on the viewfinder display enables an adaptation of the displayed image, for example a brightening in weak light. Optical image stabilizers, digital photo and film functions can be integrated (e.g. Sony DEV-50V ).

### History of product diversity

This portrait of a lady with a lorgnette (theater glass ) was painted in 1884 by the Polish artist
Anna Bilińska .

Binocular binoculars on representations before 1900 were mostly theater glasses, i.e. devices based on the Galileo telescope principle. The following prism binoculars were initially manufactured almost exclusively as monocular binoculars, for example by the company M. Hensoldt & Söhne in Wetzlar. The companies Carl Zeiss in Jena and Leitz in Wetzlar (Binocle 6 × 18, with Porro prisms, 1907) mainly produced binocular prism binoculars.

In the first half of the 20th century, prism binoculars were designed to a significant extent for the military as well as for private individuals. Wide fields of view were preferred to the detriment of the then hardly required glasses suitability. The product variety was comparatively small, and the optically better Porro prism binoculars were predominantly produced. In the last quarter of the 20th century, improvements in coating and manufacturing technology as well as the increased purchasing power of broad sections of the population led to an increase in the range of high-quality roof prism glass in universal and compact design. Since the beginning of the 21st century, the differences in quality between the two types of high-quality devices have practically no longer existed, especially since roof prism glasses with high-quality lens systems and effective phase correction coatings are now on the market, which are also often suitable for close range. In addition, due to the increased number of people above the age of 45 who wear glasses and often have affluence, greater importance is attached to the suitability of glasses and generous diopter adjustment, the best possible optical indicators, lightness and compactness as well as other ergonomic aspects. Finally, image stabilizing devices that have been around since the end of the 20th century have also been strengthened and come on the market in various forms of construction.

At least around 1980 to 1990 it was common for cheap binoculars to include plastic protective caps for lenses and eyepieces with the usually hard case made of lacquered cardboard and two orange filters that could be attached to the eyepiece. These absorb the short-wave half of the spectrum, which is more strongly scattered by haze (and air) in order to achieve better, albeit practically monochrome, distance vision. A long oval double cap that extends over both eyepieces and is threaded with two slits on the lanyard can protect binoculars that hangs on the front of the chest when standing and walking from dust from above and can also be used when lifting up to the eyes quickly removed from the second hand.

Around 8 million new binoculars are sold on the world market every year. Up to several 100,000 copies of individual device types in the high-end area (e.g. hunting glasses) can be sold in the course of a production cycle. Binoculars from well-known brands are considered high-quality and potentially durable consumer goods. Customers often keep them even if they buy new binoculars, which gives the glasses a collector's value and a certain lasting value.

## Functional principle, parameters and quality properties

The three essential optical components of prism binoculars are

• 1. the lens, which nowadays consists of two to five lenses,
• 2. the prism system with two or three prisms as well
• 3. The eyepiece, which today usually consists of three to six lenses.

In an intermediate plane, the lens creates a doubly inverted image - upside down and reversed, i.e. rotated by 180 °. The prism system rotates the image by 180 ° and straightens it without changing its size. The eyepiece works as a magnifying glass with which the intermediate image can be seen enlarged. If the focus is correct, the light rays exit the eyepiece in parallel, and the observer has the impression of being able to look into the distance in a relaxed manner.

### Magnification number and lens diameter

The magnification and the lens diameter are on practically all binoculars (here: 7 × 50).
The field of view size in Anglo-Saxon countries is often given in
"feet per 1000 yards" instead of in ° or in "m per 1000 m" (here: 372 ft. At 1000 yds., Corresponds to 124 m per 1000 m or 7.1 °) .

The most important and usually also described parameters of binoculars are the magnification and the lens diameter . For example, the marking 10 × 50 means that there is a 10-fold magnification and an objective diameter of 50 mm. The magnification is the quotient of the focal length of the lens and the focal length of the eyepiece . A 10X magnification means that the image will appear 10 times larger than what you see with the naked eye. The lens diameter determines how much light is received by the binoculars and is therefore partly responsible for the image brightness, which is also influenced by the magnification. Image brightness is proportional to , where D is the lens diameter and v is the magnification. ${\ displaystyle (D / v) ^ {2}}$

Some binocular manufacturers tend to use established key figures (e.g. 8 × 32, 10 × 50) even if the lens diameter is a few millimeters smaller in individual cases. The magnification of the image seen is also not so clearly defined:

• First, the actual magnifications between binoculars will vary slightly around the stated values,
• second, they change somewhat with the object distance (apparent lower magnification at small distances) and
• Thirdly, the respective distortion changes the magnification insofar as the pillow-shaped distortion typical of binoculars increases the magnification factor somewhat towards the edge.
• After all, with binoculars of the classic Porro design, one has the subjective impression that the magnification is somewhat lower than that of roof- top binoculars with the same magnification and a smaller lens distance, which is particularly pronounced in the vicinity; this so-called lilliputism effect is the result of a different perception or cerebral offsetting of the more stereoscopic image and does not apply to monocular viewing.

The bundle of rays that passes vertically through the objective into the binoculars has the effective diameter of the respective objective. The effective diameter is sometimes slightly reduced by an entry aperture behind the lens or even further inside the device. These diaphragms are mostly used to suppress imaging errors in the edge area. The diaphragm or lens mount that defines the diameter of the beam defines the so-called entrance pupil . The quotient of the effective lens diameter and magnification (for example 50 mm / 10 = 5 mm) determines the diameter of the beam that leaves the eyepiece and hits the eye. This diameter of the beam is known as the exit pupil . It is the image of the entrance pupil and is directly proportional to the size of the objective (with unchanged magnification!). If you hold the binoculars at a certain distance from the eye, the exit pupil can be seen as a bright circle of light “floating” in front of the eyepiece. Since the pupil of our eye has a pupil opening of 2 to 3 mm in daylight and around 6 mm in the dark (maximum 7 to 8 mm, in advanced age often only up to 5 mm), the size of the binoculars exit pupil is sensible either 2 to 3 mm or about twice as large. Unfortunately, this sensibility is mostly disregarded. There are far more binoculars with the larger exit pupil in use than with the smaller one. The doubling of the exit pupil means that the binoculars have at least twice the weight that is carried around without essential use on walks or hikes that take place during the day. The excessively large exit pupil is only an advantage if you cannot hold the binoculars steadily in front of your eyes (for example on a swaying ship). An exit pupil below 2 to 3 mm leads to a significant reduction in perception due to the darker image. For all-round binoculars (without image stabilizing additive) that can also be used at dusk and on clear nights (star observation), 6 to 9 times magnification and a maximum of 44 mm objective diameter (9 to 12 times and a maximum of 60 mm for strong people of younger and middle age) are recommended.

### Luminous intensity, twilight factor and transmission

Two further key figures are used to characterize the suitability of binoculars with low light intensity:

• The light intensity (more precisely: geometric light intensity) is proportional to the square of the exit pupil. It is given as a dimensionless number resulting from the square of the exit pupil measured in mm.
Example 10 × 50 glass: light intensity = = 25${\ displaystyle \ textstyle (50/10) ^ {2}}$
• The twilight factor is the square root of the product of magnification and lens diameter. It is also given as a dimensionless number.
Example 10 × 50 glass: Twilight factor = = 22.36${\ displaystyle \ textstyle {\ sqrt {10 \ cdot 50}}}$

Although both values ​​are usually specified as dimensionless numbers, they are actually not dimensionless, but rather depend on the unit used for the entrance pupil diameter, which must therefore be specified for clear identification. Both key figures are also greatly simplified performance parameters that emerge from the borderline cases of far more complex models of telescope performance .

Effective light intensity and effective twilight factor result if the dependency on the transmission (light permeability) of the glasses used and on their surface treatment (mirroring, coating) is taken into account.

The transmission of the respective optical system can only be measured using instruments. It indicates what percentage of the incident light radiation leaves the eyepiece after passing through the entire optics. This percentage is different for different wavelengths, which is why binoculars, depending on how they are manufactured, can leave a color impression that differs slightly from nature. High quality binoculars achieve transmission values ​​of over 90 percent in the range around 600–700 nm and between 80 and 90% around 450–600 nm. Older binoculars up to around the middle of the 20th century had a maximum transmission of around 70% due to a lack of coating and thus showed a slightly darker picture compared to modern glasses. - In today's advertising materials, significantly higher transmission values ​​are simulated (e.g. "99%"), but these only apply to individual lenses or prisms, but not to the entire system.

### Real and apparent field of view

The real field of view is specified either in degrees of angle (°) or as a field width at a distance of 1,000 m. 1 ° corresponds to approximately 17.5 m field width at a distance of 1000 m.

Exact calculation: Half the field width is the product of the tangent of half the viewing angle and the distance.

Calculation for 6 ° viewing angle: tan 3 ° × 1000 m = 0.0524 × 1000 m = 52.4 m; the field of view is about 105 m wide at 1000 m (2 × 52.4 m).

The real field of view of binoculars decreases with increasing magnification and ranges from around 3 ° (around 50 m per 1000 m, typically for 18 to 20 times magnification) to around 9 ° (around 160 m per 1000 m, typically for 6- up to 7x magnification). The field of view cannot be enlarged arbitrarily, the aberrations occurring at the edge and the dimensions of the prisms and ocular lenses set a practical limit. Binoculars with a larger field of view often show the edges blurred and distorted. They were and are still occasionally produced despite the marginal defects, insufficient glasses suitability, high weight and susceptibility to failure. Examples were the Leitz Amplivid (12.1 °) and the Zeiss Deltar 8 × 40 (11.3 °) as well as the Leitz Trinovid 6 × 24 (12.1 °). Field of view sizes of this kind are advantageous if the object to be observed moves quickly across the direction of vision (sport, hunting) or if the field of view to be observed is large (theater stages). This gap in the market is filled by the Bushnell XtraWide series (17 °).

Even with binoculars that are suitable for use with glasses, people who wear glasses cannot use the entire field of view that can be viewed without glasses. Far-sighted people are affected in a different way and sometimes more than near-sighted people. This cropping of the field of view should be tested individually, as the (anyway only approximate) field of view size for glasses wearers is rarely mentioned in the advertising text.

The apparent field of view (also known as the apparent angle of view ) relates to the angular diameter of the field of view limited by the field diaphragm as perceived by the observer when looking through the eyepiece. It is roughly the product of the real field of view and the magnification factor. With 6 ° real field of view and 10x magnification, the apparent field of view is about 60 °. This simple multiplication only approximates the exact apparent field of view size, but takes into account the widespread pincushion distortion. The more recent standard ( ISO 14132-1: 2002) requires a modified (again trigonometric) calculation (in the case of the absence of any distortion), which usually leads to an apparent field of view which is usually 2–4 ° smaller; for this reason older and more recent information for the same binoculars may vary and comparisons between different manufacturers must take into account the basis of the calculation.

If the apparent field of view is too small, the observer can get a “tunnel vision” or “keyhole view” impression, but the limit below which this impression arises is subjectively different (usually between 55 ° and 60 °). Eyepieces with an apparent field of view of over about 60 ° are called wide-angle eyepieces.

### Depth of field

The depth of field depends primarily on the magnification of the binoculars. Weak magnifications make it easier to see objects at different distances at the same time, while higher magnifications require frequent refocusing. The depth of field is reduced quadratically with the magnification, i.e. H. 10 × binoculars have about half the (7² / 10² = 0.49) depth of field of 7 × binoculars.

Secondly, the object distance is of great importance: at a distance of 2 m, the subjective depth of field for binoculars with 7 to 8 times magnification is around 10 cm, at a distance of 10 m around 1 m, and from around 25 m you can see up to infinitely everything reasonably sharp (the fixed focus binoculars are based on this effect). However, the depth of field is better in young people with a high level of accommodation in the eyes than in older people, as they can, mostly unconsciously, adjust the shape of their eye lens somewhat. In addition, the subjectively perceived depth of field also depends on the light intensity and also somewhat on the type of object.

Thirdly, other factors also have an influence. The exit aperture of the binoculars or the eye pupil (depending on which diameter is smaller) play a role, since a smaller effective pupil, similar to the aperture in a camera, increases the depth of field. With binoculars with “tunnel vision”, the eye pupil can open a little too wide during the day because of the dark edge, which can reduce the depth of field and also cause glare. Conversely, a wide-angle eyepiece closes the pupil of the eye slightly, thereby increasing the depth of field.

### Image errors

Every optical system, and thus every binocular, has physically caused imaging errors that cannot be eliminated, but only reduced by connecting several lenses made of different types of glass one behind the other. The most important aberrations are the field curvature , the spherical aberration and the color errors . The distortion (distortion) is as consciously used, a special case.

The curvature of the image field means that a flat subject cannot be focused over the entire surface at the same time, as it is depicted on a curved surface. The corrective measure is called flat field optics , with which this error can be reduced to such an extent that it no longer has a disruptive effect.

A spherical aberration , also known as aperture error, occurs in lens shapes in which light rays that strike the center of the lens have a different focus than light rays that strike the lens towards the edge. The result is a sharp core image, which is overlaid by a blurred one. This leads to a "blurring effect", i. H. to a low-contrast, soft and somewhat blurry-looking, albeit sharp image. The effect occurs particularly with wide-angle eyepieces. This error can be corrected by using (expensive) aspherical ocular lenses .

Achromatic lenses are used to minimize the color errors (chromatic aberration) that can be seen as a color fringing (especially at light / dark transitions) in every lens due to the refraction of light . These consist of two lenses of different types of glass (i.e. glasses with different Abbescher numbers ), which have the effect that the red and blue spectral components have the same focal length (elimination of the longitudinal aberration ). For some time now, binoculars have also been on the market in which a third color ( apochromat ) or even a fourth color (super achromat) have the same focal length. These complex corrections can also reduce the lateral color aberration (color fringes at the edge of the field of view).

Other imaging errors that occur are coma (asymmetry errors ), astigmatism (lack of dots) and vignetting (darkening of the corners).

In general, special types of glass and complex coatings are combined to reduce disruptive imaging errors, which makes high-quality glasses very expensive to produce. Furthermore, some optical performance data, such as the field of view size, are deliberately limited in order to keep imaging errors at the image periphery low.

### Distortion

Distortion is the deliberately calculated deviation from a true-to-scale image when calculating and manufacturing the lenses, resulting in a targeted slight distortion. They bend straight structures at the edge of the picture outwards at the ends (pillow-shaped distortion). This means that the enlargement factor increases by a few percent towards the edge. (The opposite phenomenon, barrel distortion, causes an effect due to a decrease in the magnification factor to the outside, such as that produced in photography by the fisheye lens ).

While a pillow-shaped distortion is generally undesirable for camera lenses and photographic images, many manufacturers intentionally design their binoculars so that the optical image appears more stable and steady when the glass is swiveled, which is particularly important with a large field of view. On the other hand, this causes a distortion in the case of straight structures at the edge of the picture, which is particularly important when viewing tall buildings from below, e.g. B. church towers can have a disturbing effect, since the upper parts of the tower then occupy an unnaturally large-looking area of ​​the picture. If there is no or little distortion, the lines remain straight, but an observer who swings his binoculars often gets the impression that he is moving in front of bulbous mirrors, which is sometimes perceived as uncomfortable. This optical phenomenon is known as the globe effect , because the human brain interprets the visual information during the pivoting movement as if the pixels were moving on a globe surface.

So if you often observe with your binoculars in places with long, straight structures (cities, forests), low-distortion glasses may be preferred, even if the globe effect is not perceived as disturbing. If it is more important to look at a panorama while panning the glass, a stronger distortion should be desirable. Japanese models are often manufactured with relatively little distortion and advertised as "distortion-free" (e.g. Nikon High Grade and Action series, Pentax 8x32 DCF SP , Kowa Genesis 33 ). In the past, European manufacturers also used largely distortion-free production, while the leading representatives ( Leica, Zeiss, Swarovski , etc.) now factor in distortion, but adapt the strength of the distortion to the respective market needs and design more or less.

### Glass types and tariffs

The choice of glass types and the type of coating (anti-reflective coating) of lenses and prisms have a strong influence on the light intensity and contrast . Contrast is the difference in luminance between adjacent light and dark surfaces. Subjectively, high contrast is perceived as more brilliant and apparently sharper than low contrast, but it can come at the expense of color fidelity. High-quality types of glass and coatings help to reduce unwanted stray light, backlight reflections and color fringes. Frequently used types of glass for prisms are the crown glasses BK7 (borium crown glass) and BaK4 (barium crown glass), furthermore the glass type SK 15. BaK4 prisms allow a high resolution of details and a bright and true-to-color image. However, with these and other types of glass, color fidelity, dispersion , geometric shape, weight, quality variance in production and manufacturing costs are interrelated in a complex manner, so that different types of glass and constructions may be used as the most suitable for the respective purpose in the future. The special glasses or the respective combination of optical measures have company-specific names for marketing reasons. Kowa speaks of XD lenses, Nikon of ED glass, Swarovski and Leica of HD, Steiner of XP optics and at Zeiss they are called FL glasses.

In optics, coating or anti-reflective coating is an anti-reflective coating that is achieved by vapor deposition of metal oxides and fluorides on lenses and prisms. These substances have a low refractive index. Without compensation, there would be considerable light reflection on the glass surfaces and low light transmission (degree of transmission). While single coatings improve the transmission especially in the yellow light spectrum, in which human eyes are particularly sensitive, multiple coatings reduce the degree of reflection of the glass over a large part or even the entire visible wavelength range. Today almost only multi-coating is used. Depending on the intended use of the binoculars, however, different tariffs are preferred, for example for hunting glasses a maximization of the transmission in the wavelength range around 500-540 nm, which is important for observation in twilight. Furthermore, depending on the tariff, the character of the image seen in the binoculars under normal daylight can Appear “warmer” or “colder” and either more or less contrasting. Depending on the application, the coating is also optimized for maximum color fidelity, for example for glasses especially for bird watching.

Additional important properties that have recently been advertised are a high level of scratch resistance on the outside of the lenses and the lotus effect , which allows water and dirt particles to roll off easily. The latter has brand-specific protected names, such as LotuTec at Zeiss, AquaDura at Leica, RNP Raindefender Nano Protection at DDoptics or Nano-Protection at Steiner.

## Construction of the optical system of the prism binoculars

Porro prism system in 3D representation
Schmidt-Pechan roof prism system in cross-sectional drawing and in 3D representation
Abbe-König roof prism system in 3D representation

### Design and manufacturing principles

In prism binoculars, in addition to lenses, the prisms are essential optical components , which either through total reflection or through mirror layers ensure that the image seen is upright and correct. At the same time, this shortens the overall length of the optical system. Different designs influence the shape and size of the binoculars and the properties of the overall optics.

The Porro prism binoculars (Zeiss patent 1893) are based on the Porro prism patented by Ignazio Porro in 1854 . This deflects the light rays coming out of the lens several times and rotates the image by 180 °. As a result, the image that is initially reversed from the lens in the intermediate image plane is seen correctly and upright. Two Porro prisms are required for each beam path .

An external feature of Porro prism binoculars is the wide and short design with mostly widely spaced lenses, which improves stereoscopic vision. On the other hand, close-up vision is more difficult with classic Porro prism construction. With small lens diameters (usually up to 21 mm in diameter), the lenses are sometimes offset inwards (reverse Porro prism construction). This allows weight and volume savings, but is at the expense of the stereo effect. On the other hand, close observation is improved (currently only implemented in the Papilio series from Pentax ).

The roof prism binoculars (Hensoldt patent 1905) use a roof pentaprism system for each beam path for 180 ° rotation of the image. Roof prism systems exist in different designs; the most well-known are

• the Schmidt-Pechan prism system with a total of 2 prisms (a Schmidt prism and a Pechan prism); it requires a mirror coating due to the lack of total reflection on a surface, but allows a short and compact design and is probably included in the majority of modern roof prism binoculars;
• the Uppendahl prism system with a total of 3 prisms, which results in a somewhat longer construction than the Schmidt-Pechan prism system and is therefore used less often (e.g. in the Leica Geovid binoculars with laser rangefinders);
• the Abbe-König prism system (Dialyte prism system) with a total of 2 prisms, which does not require mirroring and thus enables higher light transmission; it is z. B. used by Zeiss in the Dialyt 8x56 and in the larger Victory glasses from 42 mm objective diameter, and causes a rather long construction.

There are other types of prism with special properties that are apparently not currently used anywhere in binocular binoculars

• the Sprenger or Leman prism ; It is made from a single piece and was previously installed by Carl Zeiss Jena in theater glasses based on prisms and in subcompact binoculars (e.g. Theatis 3.5x15 , with 11 ° real field of view and close focus up to 50 cm!);
• the Möller prism system , which consists of 2 prisms, has high transmission values ​​and enables lenses to be placed close together; it was previously used by JD Möller Optical Works Wedel .

Outwardly, the corresponding binoculars are reminiscent of Porro prism binoculars.

As a result of the silver-plated surfaces on a mirror surface, which Schmidt-Pechan prism systems and Uppendahl prism systems require, they allow less light to pass through than Porro prism glasses with otherwise the same treatment. Interference effects between the two beam paths also lead to a poorer resolution in the case of very fine structures in all roof prism systems, but this can largely be remedied by the vapor deposition of a phase correction coating (introduced in the 1990s). In addition, the highest manufacturing precision is required for roof prism systems in order to achieve a high-quality image.

The advantages of roof prism binoculars are usually mentioned:

• Close focus is easier to achieve, even for larger binoculars;
• no lilliputism effect (i.e. apparently lower magnification, especially at close range);
• Inner focusing allows good water and dust tightness (also possible with complex Porro binoculars, but often leads to difficult adjustment);
• lower weight and smaller dimensions and therefore sometimes better stability;
• better suited for small hands.

The advantages of porro prism binoculars are usually mentioned:

• In the low-price sector, brightness and optical quality are almost always better;
• better spatial image impression in the middle distance range around 10 to 100 m;
• In principle, a larger field of view can be achieved (which, however, must be reduced again for full spectacle suitability);
• when seated, the glass can be placed on the lens mount and quickly brought to the eyes;
• People with large hands perceive handiness better.

These general and different advantages and disadvantages in manufacturing and production costs are the reasons why even today most manufacturers offer binoculars based on the Porro prism principle as well as the roof prism principle and find corresponding customers. Due to their construction, large binoculars are practically only manufactured in Porro construction, real compact binoculars only in roof prism construction.

### Focusing and fixed focus systems

An important function is the focusing of the object to be observed. The lower limit is the minimum distance (close focus, near point, near focus), the upper limit corresponds to infinity, with a certain overstroke of approx. 3 to 7 diopters being provided so that even (moderately) myopic people can still focus. For farsighted people, the effective minimum distance is slightly larger. With the devices currently on the market, the adjustable near point varies between 50 cm and over 20 m. A high magnification often requires a greater minimum distance, but the construction of the device, the lens calculation and configuration and the types of glass used are also decisive. With older binoculars and still today with many Porro prism binoculars, the minimum distance is 3 to 10 m. For roof prism binoculars, the near limit offered is now often 1.5 to 2.5 m, sometimes even up to 1 m ( Vixen : Atrek , Apex Pro and Foresta series, various Fujifilm glasses), but that is observation with less than about 2 m exhausting and tiring, as the eye pupils have to be directed inwards with “cross-eyed”; In addition, the binoculars have to be placed a little closer. Above all, however, the two eyes perceive very different image sections as a result of the parallax , which the viewer's brain tries to convert into a uniform image. Only binoculars with Porro prisms that are installed the other way round (Pentax Papilio) allow relaxed viewing up to around 50 cm, which enables, for example, the enlarged observation of insects.

There are also binoculars on the market without distance adjustment. They are often assigned terms such as fixed focus , permafocus or even autofocus (not to be confused with automatic focusing , which occurs in cameras, for example). All optical components are mutually fixed for the usually sufficient distance range of a few meters (from around 20 m, for example in the Steiner binoculars with “Sports Auto Focus”) to infinity. There is only a double-sided diopter adjustment on the eyes, which is missing in cheaper binoculars.

Fixed focus glasses are used, for example, to observe sporting events that take place at a greater distance (football games, horse races, sailing regattas). Binoculars for nautical purposes ("marine glasses") also often have a fixed focus, as the objects to be observed are mostly at a greater distance. An additional advantage, especially for use at sea, is that they can simply be made watertight because no external adjustments have to be made. The disadvantage is the relatively small magnification (4 (rarely) to 7 times), which results from the required higher depth of field.

A certain additional distance adjustment takes place in the eyes of the observer, which becomes noticeable through fatigue after prolonged observation. Younger observers who still have a good ability to accommodate their eyes have an advantage.

### Variable magnification binoculars

Several manufacturers also offer binoculars with variable magnification. There are two principles here:

• Binoculars with continuous adjustment according to the zoom principle (mostly for a partial area between about 5x and 20x magnification)
• Binoculars with two fixed settings , between which you can switch (usually from 7 to 12, from 8 to 12 or from 10 to 15 times magnification).

The first category contains very different porro prism binoculars; They are offered in all weight and light intensity classes, from 180 g (e.g. Eschenbach Vektor 5-15x21 ) to over 1000 g (e.g. Nikon Action 10-20 × 50CF ). Compared to binoculars with fixed magnification, binoculars with zoom magnification have more lenses and thus - especially in inexpensive models - both higher light losses and annoying color fringes . Furthermore, due to the longer construction of the eyepiece, the field of view and close focus are usually significantly worse than with glasses with a fixed magnification, even at the lowest magnification. At the highest level of magnification, the exit pupil is small and the visual comfort and twilight vision are therefore severely restricted. In addition, from about 12x magnification, the camera shake is very annoying without using a tripod. Due to these restrictions, users with high optical demands rarely use zoom glasses.

The second category often includes high-quality roof prism binoculars; the products currently on the market ( Leica Duovid series, Leupold Golden Ring series) weigh around 600 to 1250 g. There are construction-related restrictions (e.g. low close focus), albeit to a lesser extent, with these too.

The functional principle of the zoom glasses: The synchronization takes place on both sides via a mechanical coupling, often in the form of a flexible metal strip, which is guided in a non-compressive manner in a rail along the eyepiece bridge (which also enables synchronous focusing via the central drive). The magnification is usually set using a lever that moves lens groups in both eyepieces via the aforementioned coupling and a type of worm gear .

### Binoculars with image stabilization

Somewhat blurring-free can only be observed up to about 7 to 12 times magnification, and this usually only at rest, for example not under physical exertion (e.g. on a mountain hike) or on a ship.

The manufacturers Canon (models with IS in the name), Nikon (StabilEyes models) and Fujinon (Techno-Stabi models) offer binoculars with switchable electronic image stabilization , which can significantly reduce blurring and stabilize their function, especially when used hands-free Camera lenses is known. The disadvantages include battery consumption, larger volume, greater weight, mostly increased minimum distance to the object (often between 3.5 and 6 m, but with newer models also 2.5 and 2 m) and a smaller field of view than normal. They work like normal binoculars without or with empty batteries.

Carl Zeiss offers binoculars, the 20 × 60S (S as the symbol for image stabilization), in which image stabilization is purely mechanical using a gimbal-mounted, vibration-damped prism system. So no batteries are necessary here. The various systems differ in terms of their robustness and the type of correction: some systems hold a stable image when the binoculars are swiveled, others enable moving objects to be tracked and can better compensate for the trembling and shaking, so that especially with high magnification (20- fold) the finest details become visible.

There are also binoculars with gyro-assisted, purely mechanical image stabilization (Fujinon, Fraser-Volpe), some of which are also offered with night vision options.

## Choice of suitable binoculars

### Terms and Conditions

Size comparison between "standard" binoculars 8 × 30 ( Porro prisms ) and compact binoculars 8 × 20 ("straight" roof prisms )
First compact binoculars (8 × 20) from Zeiss, Oberkochen; Above: tape attached by the user to replace the missing central drive; below: folded in a zigzag shape
8 × 20 compact binoculars with central drive

When purchasing binoculars, one size that is often neglected but important is the exit pupil, which in principle does not need to be larger than the eye pupil. Those who use binoculars mostly during the day, as usual, get by with a significantly lighter and less bulky instrument; he does not have to wear a glass of size 6 × 30 (or 8 × 30) that has been mutated from the military standard to the general standard. In April 1969, the Zeiss company, Oberkochen, launched binoculars measuring 8 × 20 (exit pupil 2.5 mm) that weighed just over 100 g. Since the distance setting had to be made individually on both halves (no central drive), it could be folded in a zigzag to a width of only 70 mm (total volume 90 mm × 70 mm × 26 mm) and put in a shirt pocket. For comparison, the “standard” Deltintrem glass from Zeiss, Jena from the same period: 165 mm wide, more than 500 g. Today, such compact binoculars also have a central drive, are a little heavier and, when folded, are also a little wider. However, the concept has not prevailed until today, possibly because of the many cheap offers (from 10 euros) with only minimal quality, although there are of course enough products with high quality (from 100 euros).

The “standard” glass with an exit pupil of around 5 mm is suitable as night glass for use at dusk. An exit pupil of 7 mm is only suitable for the completely dark-adapted eye of a person who is not too old (for example 7 × 50). The use of a glass with very high magnification is also an exception, for example when observing wild animals. It is practical to use a tripod, which means you can go beyond 12x magnification (monocular with very high values).

The preference for a high magnification is - unlike that of a high exit pupil - of no decisive importance. High magnification allows smaller, distant objects to be better recognized, but this comes at the cost of greater image unsteadiness, a smaller field of view, a smaller depth of field and a lower light intensity or greater weight. The disadvantage of only 6 or 7 times magnification compared to 10 times magnification is in many cases bearable in view of the advantages traded in: A 6.5 times magnification means that a bird 100 m away appears as large as if it were still there about 15 m away, while a 10x magnification looks like you're 10 m away. This means that subjectively in the first case you feel “only” 85 m closer to the object, in the other case about 90 m closer.

People who wear glasses must pay attention to binoculars that are suitable for glasses if they do not want to take off their glasses every time they observe (this is made easier by using a glasses strap). If the binoculars are to be used with or without glasses, it is advisable to carry out a detailed test before buying to determine whether all settings (diopter adjustment, eyepiece or eyecup adjustment, infinity position and close-up position when focusing, visibility of the entire field of view) are both as well as can be achieved satisfactorily without glasses.

When purchasing binoculars, in addition to the intended use, the personal situation should also be critically considered: a rather light person can be burdened with binoculars that a strong person perceives to be too light because it is prone to blurring. The size of the binoculars and the usability should also match the size of the hand.

### Selection according to intended use

• If you don't pay attention to image quality and longer use , but just want to chat, you should buy inexpensive binoculars, possibly even with variable magnification . Binoculars with zoom magnification are also available for this largest number of users worldwide. Zoom lenses are offered in all weight and price classes, but are especially in the compact and subcompact range around 180 to 310 g (e.g. Eschenbach Vektor 5-15 × 21 , Pentax 8-16 × 21 UCF Zoom II , Nikon Travelite 8-24 × 25 CF ) useful. For design reasons, binoculars with variable magnification can in principle never achieve the optical performance of good glasses with a fixed magnification factor: Among other things, there are always reductions in the size of the field of view, in the close focus and in visual comfort at the highest magnifications (very small exit pupil, sometimes only 1 mm).
• Compact binoculars, also known as pocket binoculars, are intended for the handbag or even shirt pocket and are sometimes offered in high optical quality with the parameters 6 × 18 to 10 × 25 ( Leica , Zeiss , Swarovski , DDoptics , Nikon , Pentax , etc.). They usually weigh between 180 and 300 g and are the only binocular prism binoculars that you can "always have with you". However, they are not suitable in twilight and have a somewhat restricted field of view. Many, especially in the low-price sector, are not suitable for people who wear glasses. Many people find it difficult to keep them tremor-free due to their small mass, which is why a rather low magnification of 6 × to 8 × is advantageous. The Vixen Arena 12 × 23 (and some zoom lenses) is an exception with an exit pupil smaller than 2 mm. For compact binoculars for visiting theaters, churches or museums, a maximum of 5x magnification is recommended (e.g. for Nikon 5 × 15DCF Titan ).
• The "standard" binoculars of around 8 × 25 to 10 × 44, which weigh between 300 and 900 g, are suitable as universal binoculars for the day on hikes, for landscape, game and bird watching and for twilight. The higher weight is only a compromise for not having to compromise on any other requirements. Basically, both roof prism glasses and Porro prism glasses come into question, the latter tending to be somewhat heavier and mostly unsuitable in the close-up range, but are characterized by a good price-performance ratio in terms of optical image quality. Binoculars up to around 500 g can still be worn on the trouser belt, for which some manufacturers offer suitable belt pouches.
• High quality glasses with variable magnifications can be suitable for specific applications. However, due to the necessary optical compromises, many well-known manufacturers do not offer zoom glasses, but at most roof prism glasses with a switch mechanism for two magnifications ( Leica , Leupold ). The flexibility in magnification is always at the expense of maximum optical performance and entails restrictions in the size of the field of view and in the close-up focus; the weight is also higher than that of a glass which has the highest fixed magnification number. The switching mechanism can be perceived as too sluggish for some situations; a previous test in your own area of ​​application is recommended. In addition to the advantage of different magnifications, switchable lenses also have several design-related disadvantages.
• The above-mentioned universal binoculars are suitable for observing animals from day to dusk, as well as high-quality roof prism glasses with magnification switch. For precise bird identification, the extreme minimization of color errors is essential, which may, however, reduce the contrast somewhat or cause a slight sensitivity to back light. For animal observations with frequent twilight and night use, e.g. B. as hunting glasses , only bright variants are suitable. Frequent characteristic values ​​here are 7 × 50 (calm, bright panorama image) over 8 × 42 and 8 × 56 to 9 × 63 (better detail recognition, but 870 to 1500 g in weight). Hunting glasses are also offered in combination with battery-operated laser rangefinders (for a distance of 10 to 1200 m with additional ballistic information).
• The range observation is an application that was basically earlier already realized (eg. As with the use of sprinkler-Leman prisms since the 1920s), but plays only recently again as a selling point a role. With glasses like this, insects, flowers or lizards can also be seen and observed in detail. Various manufacturers of high-quality binoculars ( Kowa , Pentax , Swarovski , etc.) offer values ​​of up to 1.5 m as the minimum distance to the object , whereby the ergonomics should be checked in individual cases, as the left and right beam paths send considerably different images to the eyes . Thanks to a modified design, the Porro prism-based Papilio models from Pentax allow a very comfortable close-up observation up to 50 cm. This enables even the inspection of ancient coins in the archaeological museum or the deciphering of small inscriptions and also the viewing of model railroad systems , where the narrowing of the visual rays increases the realistic impression. Since the depth of field decreases rapidly with increasing magnification, especially in the close-up range, enlargements for the close-up range are generally only recommended up to a maximum of 7x.
• If you often want to enjoy a panoramic view of the landscape, you should pay attention to a large field of view and calm pan image. In order not to get a "rolling" globe effect when panning, binoculars with adequate pillow-shaped distortion should be selected. However, since this property is usually not explained in more detail in the advertising material, it is advisable to seek specialist advice or to consult test reports. Binoculars with a large field of view tend to have aberrations in the edge area, which are, however, greatly reduced in top-quality devices. In addition, large fields of view require a somewhat larger construction, which is based on the installation of larger prisms or the use of lenses with a longer focal length. Well-corrected roof prism binoculars are now available with 7x magnification for visual ranges of a maximum of about 150 m per 1000 m, with 8x magnification up to about 140 m, with 10x magnification up to about 120 m and with 12x magnification up to about 100 m. Since the area of ​​the field of view increases by the square of the visual range, it is already twice as large at 141 m as at 100 m. Larger viewing distances are very rarely offered, e.g. B. the Bushnell XtraWide 4 × 30 with 300 m per 1000 m (fixed focus, not suitable for glasses wearers), or the Nikon Action VII 7 × 35 with 163 m per 1000 m. Many lenses with a large field of view also offer a "calm" image (see following section ).
• A steady and stable image is best achieved without a tripod with a magnification of up to about 7x. This also enables a wide field of view and an impressive depth of field. Another important prerequisite is that there should be as few kidney beans as possible (black shadows on the sides if the view is not exactly centered). Due to the low magnification, binoculars with small lens sizes up to 21 mm still provide a sufficiently bright image during the day; with occasional twilight use they should be at least 35 mm and with professional twilight use or night use at least 42 mm. The binoculars should weigh around 250–400 g for petite people and around 400–600 g for medium-strength people, in order to lie calmly in the hand on the one hand and not to appear tiring when used on the other. Binoculars with these magnification and weight requirements are rather rare ( Pentax Papilio 6.5 × 21 with 290 g, Vixen Foresta 6 × 32 DCF with 450 g, Leupold Katmai 6 × 32 with 515 g, Steiner Navigator 7 × 30 with 520 g (Marine glass with fixed focus)). In addition to low magnification, a wide field of view and adapted weight, other factors are also important, in particular high-quality corrected optics to minimize the scattered light, an exit aperture of at least 4 mm and comfortable handling. An individual test is always appropriate.
Modern large field glasses in naval use
• For marine glasses (binoculars for nautical purposes and in water sports), a rather low magnification with a large exit aperture (7 × 30 to 7 × 50) is recommended, which facilitates observations on fluctuating ground and at the same time makes the glass suitable for twilight through to night. A higher weight is not a disadvantage on board, because unlike when hiking, the glass does not have to be carried over long distances. Glasses with a high contrast are considered practical. Porro prism binoculars that are specially designed to be watertight are often offered, some with an additional mirrored (battery-operated) compass and a graduated scale for easy distance or size estimation (with an approximate knowledge of the other size). Special features can be a thread for attaching polarization filters against light reflections on the water, a waterproof and clear diopter adjustment that can be easily adjusted to different users of the ship and, to a certain extent, also serves to focus (improves water resistance compared to central focusing). Swimming straps that let the binoculars float in an emergency are practical. The military and border protection authorities have very large marine binoculars that can only be used with a tripod.
• For astronomical observations similar glasses are used under a dark country sky as for twilight vision (e.g. 7 × 50, 8 × 56, 9 × 63). However , their use is less advisable under a lightened sky , as it occurs in urban areas, because the unwanted light from the sky background is also captured and perceived more intensely by the large exit pupil. This leads to a weakening of the contrast between the astronomical observation object and the sky background. The brighter the sky, the smaller the exit pupil of the selected binoculars should be. In the city, it is therefore easier to observe with 8 × 32 or even 10 × 30 binoculars than with 8 × 56 binoculars. A size often favored by amateur astronomers is 10x50.
Since observing at an angle up into the open sky is difficult and prone to blurring, the use of a tripod is recommended . For this purpose there are also special binocular tripods or
mounts ("Binomount"), the mechanics of which work similar to that of a desk lamp. Image stabilized binoculars also bring significant relief. Some large binoculars ("comet hunters") specially made for astronomical purposes for use on a tripod are optionally available with an angled viewer. This makes it much easier to observe high objects near the zenith .
However, many zoom binoculars are not very suitable. Their often narrow field of view hardly allows star hopping ,
i.e. looking for objects over other objects using a star map. Even more problematic are the aberrations that can usually be found in inexpensive specimens , which make the stars no longer look point-like.
• Higher magnifications always require a tripod or binoculars with built-in image stabilization . The magnification at which the meaningful limit lies is individually different; While some people can still see a relatively steady image up to about 12x magnification, for others the limit is already 6 to 7 times magnification. The additional weight of image stabilized devices is between 100 and 1000 g, depending on the design; they are currently offered in the magnification range from 8x (e.g. Canon 8 × 25 IS , 500 g) to 20x ( Zeiss 20 × 60 S , 1660 g). Due to their construction, most of them are not quite as shock and vibration resistant as high-quality devices without image stabilization.

## Appendix: Qualitative self-assessment

The quality and quality assessment of a used or new device should be left to a specialist. A self-assessment can only reveal major weaknesses:

Regarding the tightness , a look through the lenses reveals dust deposits or fogging due to cloudiness.

If you recognize a few scratches , they may be tolerable in practical use; they are hardly noticeable behind the sun in the sun, but in principle lead to light deflection due to refraction, reflection, light scattering and light diffraction. The result is that the number and strength of the scratches increase in image blurring due to stray light in the binoculars, which increasingly disrupts the image impression, especially when looking at the light or over reflective surfaces.

The general requirements for mechanics and settings are summarized above in the section "Design and ergonomics". In order to check the alignment of the two optical beam paths, a distant vertical and horizontal borderline is viewed through the glass. If you close and open your eyes, you can see whether the two partial images can be easily combined into a single image. It is true that bad adjustments can be somewhat compensated for by the (often unconscious) eye position, especially with younger observers, which is usually not noticed when a device is briefly checked, but can lead to fatigue and even headaches if it is used for a long time.

To check the geometric shape of the exit diaphragm (exit pupil), look at a distance of approx. 30 cm through the eyepieces against a light background. If it is not circular or if it has gray borders, this is usually a sign of inferior quality. Some components of the glass, mostly the prisms, may then be insufficiently dimensioned and lead to vignetting (shadowing of the image at the edge of the field of view).

Spherical aberrations can be assessed by looking at a point light source, for example a bright star. Viewed in the center of the image, deviations from the point shape indicate defects in the optics, although an almost point-like image of the star can only be expected with very high-quality glasses. In the edge area, practically all binoculars show an at least slightly blurred, distorted image of the star because of the spherical aberration , which, however, hardly affects practical use.

Color neutrality and coating of the binoculars can be assessed by looking at bright white surfaces. A blue shimmer can indicate a single coating with magnesium fluoride (MgF 2 ) (only rarely with modern devices). A green or brown cast can indicate the use of inexpensive glass or cheap remuneration. Multiple coatings are characterized by weak, differently colored reflections that can be seen when looking at the lenses from the side. High quality coatings show bluish, greenish and purple reflections.

The intensely orange-red or gold-colored effect coatings often found on glasses in the lower price range and often advertised as "nocturnal" are of no objective benefit and often reduce the transmission to below 50%; they show a green tint during the day and are faint at dusk due to the low transmission. Red coating can give the buyer an initially impressive 3D impression under department store or discounter lighting, but this does not result in any objectively measurable visual improvements in nature.

## Manufacturer

Historically, today's manufacturers and suppliers developed either from the optics industry or from jewelry crystal processing and still produce the binoculars themselves today. Other providers have their origins in eyeglass lens production or camera manufacture, partly in the weapons industry. Almost all providers sell other products in addition to binoculars. Many, including well-known suppliers, are not manufacturers of the glasses themselves or only produce certain of their series themselves. Many European, American and Japanese product lines are manufactured in China for cost reasons , where local companies often operate for different brands at the same time; higher quality product lines are partly produced in Japan . There have been numerous company takeovers, mergers and changes of name over the past hundred years . Well-known manufacturers in Germany are Steiner-Optik , Bresser , Carl Zeiss Sports Optics , Eschenbach Optik , Leica , Minox , Nikon , Swarovski .

## Synonyms

At the beginning of the 20th century, prism binoculars were also known as Trieder or Trieder binoculars .

## literature

• Klaus-Dieter Linsmeier: Long-range optics in nature observation. Function, application and manufacture of binoculars, telescopes and riflescopes. ISBN 3-478-93215-7 .
• Bernd Weisheit: Binoculars for Astropractice . In: Stars and Space , Part 1: September 2007, Part 2: October 2007.
• H. Merlitz: Handheld binoculars: function, performance, selection . Verlag Europa-Lehrmittel, 2nd edition Haan-Gruiten 2019, ISBN 978-3-8085-5775-4 .
• Hans T. Seeger: Zeiss binoculars, models - features - myth. Hand-held binoculars from 1894–1919 . Publishing house Dr. Hans T. Seeger, Hamburg. ISBN 3-00-031440-7 .
• Hans T. Seeger: Zeiss binoculars, models - features - myth. Hand binoculars from 1919–1946 . Publishing house Dr. Hans T. Seeger, Hamburg. ISBN 3-00-049464-2 .
• Lambert Spix: television viewer. Binoculars for astronomy and nature observation. Oculum Verlag, Erlangen 2009. ISBN 978-3-938469-28-6 .
• Walter J. Schwab in BERGSTEIGER magazine: Binoculars for hiking and mountaineering
• Walter J. Schwab, Wolf Wehran: Optics for hunting and nature observation . ISBN 978-3-00-034895-2