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BINOCULARS: CONSTRUCTION AND TECHNICAL DATA
The optical performance of binoculars depends on the interaction of a whole range of factors. Only when each and every one of these factors has been optimized is the result a ferroglass of excellent performance. Razor-sharp and brilliant images without color distortion always bring joy to every observation.
Inferior binoculars of lower quality are not only less fun or, if the details are not sufficiently recognizable, lead to e.g. one species cannot be determined correctly: the eyes also suffer, and headaches can result from prolonged use. In addition to the quality of the binoculars and the price, the planned application of course also plays a role in the purchase decision. For this, however, it is necessary to be able to evaluate the basic technical data of binoculars. You can find out what the individual data should tell you here.
To better understand the technical facts, you should first familiarize yourself with the structure and basic functionality of binoculars. A pair of binoculars is basically nothing more than a combination of two telescopes that are connected to one another by a common focusing mechanism via a bridge. Each of these telescopes consists of an objective (the front lenses), prisms and an eyepiece (the eye pieces).
The eyepieces are moved with the help of the focusing wheel (focus = focus) towards or away from the objective. By changing this distance, the focus is on objects at different distances. It is important that the distance between the eyepiece and the objective is identical for both binocular halves. Therefore, the eyepieces of both telescopes are connected to the focusing wheel via a common eyepiece bridge (with some binoculars, each half of the binoculars must be focused separately).
Spectacle wearers with poor visual acuity in both eyes who want to use the binoculars without visual aids can adjust the distance between the eyepiece and the lens separately from just one half of the binoculars using the so-called diopter compensation. This is done either directly on one of the eyepieces or centrally on the focusing wheel.
The prisms ensure that the image is not “upside down” and mirrored. Through various diversions, they also shorten the length of the binoculars.
The basic data in the name designation of the binoculars:
Magnification and lens diameter
The first value – the magnification – is the degree by which an object appears larger or closer than when observed with the naked eye. When using 10×42 binoculars, for example, the object appears 10x closer to you than when observing without glass. Only the eyepieces are responsible for the degree of magnification. A glass with a high magnification does not have to be large! Magnifying binoculars is perhaps the most misunderstood feature. High magnifications can be useful, but the magnification itself is not a criterion for the quality, image quality or detail recognition ability of binoculars! The lens diameter, the glass material, the coating and the quality of the overall optics determine the ability to resolve small details. The disadvantages of binoculars with a high magnification are: it is very difficult to produce a calm image over a long period of time during observation and the field of view is relatively small. For free-field observations, we therefore only recommend magnifications up to a maximum of 10 times, actually 8 times is actually the better choice!
The second value, here 42, describes the diameter of the objective lenses (front lenses) in mm. The larger these are, the more light can enter the glass and the brighter the image appears (and the larger and heavier the binoculars become). One also speaks of the entrance pupil. The light intensity of binoculars (usually given as the twilight factor) and the size of the exit pupils result from the lens diameter and magnification.
The exit pupil describes the size of the image point (in mm) that hits the eyes at the end of the binoculars facing you, the eyepieces. The larger the exit pupil, the brighter the image appears. The exit pupil is the most important and easiest to compare indicator for the twilight performance of binoculars.
You should be able to see the exit pupils as a sharply delineated bright circle from a few cm away (a soft edge indicates poor quality). You can calculate the size of the exit pupils by dividing the lens diameter by the magnification. The exit pupils of a glass with the data 10×42 have a diameter of 4.2mm.
How useful a large exit pupil is depends, among other things, on the eyes and the age of the observer, because the ability of the pupils of the human eye to enlarge in the dark decreases with age: while the pupils of a young observer still have a diameter of up to can assume to 7mm, the maximum expansion is limited to only 5mm for a 50-year-old. For example, the eyes of an older nature lover can no longer fully use the light from a 6.5mm exit pupil if the pupils of his eyes are only 5mm large. A (then lighter) glass with smaller exit pupils would be a more sensible purchase because it does not “waste” light.
The exit pupil distance indicates how far away your eyes can be from the eyepieces without losing sight of the complete field of view of the lens.
With most binoculars, the distance between the eyes and the eyepieces can be adjusted by eyecups, which means that the binoculars can also be used with glasses. So that binoculars are really suitable for glasses wearers, the most important thing is that the exit pupils are moved as far back as possible (optimally 15-20mm). This ensures that the focused image is not already shown in front of the eyes. Here one speaks of real glasses eyepieces.
In addition to the traditional slip-on rubber eyecups, some binoculars now have more robust plastic eyecups that can be pushed in or out or twisted. When using glasses, the eyecups are retracted or turned inside out.
Not to be confused with the exit pupil distance!
This is the distance between the two eyepieces, more precisely: the exit pupils of the two eyepieces. The pupil distance can be adjusted by “kinking” the binoculars at the joint between the two binocular halves and thus adapted to every face shape (the user’s eye relief).
Field of view
The size of the field of view is usually given in m at 1000m or as an angle. If the field of view is e.g. 118m / 1000m so you can see a 118m wide picture in a kilometer distance. You can easily calculate the size of the field of view by multiplying the angle by 17.45. You can calculate the angle accordingly by dividing the size of the field of view by 17.45.
The size of the field of view depends on the interplay between magnification and lens diameter and the construction of the binoculars. With the same construction, the size of the field of view decreases with increasing magnification, while larger lenses, on the other hand, enlarge the field of view.
The larger the field of view, the easier it is to survey a large area or to track a moving object.
The twilight factor serves as a standardized value for comparing the performance of optical devices with regard to the detail recognition, especially under poor lighting conditions
The twilight number is calculated from the square root of the product of magnification and lens diameter. So with 10×42 binoculars: 12×50 binoculars have a twilight factor of 24.5. The whole thing then means arithmetically that with 10×42 binoculars (twilight number 20.5) you can still see an object at a distance of 205m, with 12×50 binoculars (twilight number 24.5) you can still see them at a distance of 245m. The twilight factor of the binoculars alone is not sufficient to assess the twilight performance of binoculars; the size of the exit pupils is also used for this.
Focusing area / close focus
The design-related options do not allow binoculars to focus at distances from 0 meters to infinity. In order to be able to see from a long distance, which is the primary task of binoculars, the possibility of close focus is limited to a few meters. If, for example, a pair of binoculars has a close-up range of 3.25 meters, you can no longer focus on objects that are closer. The close-range data of binoculars mainly play a role for those who like small and close objects, e.g. Butterflies, want to watch. However, the close range should not be too small in many other applications either, think e.g. to a bird by the wayside! For others, on the other hand, the close-up area plays a subordinate role, here a glass with Fix-Focus may be more comfortable:
Fix focus: Some binoculars do not have a focusing device at all. They are set once and are then always in focus from a certain close range to infinity. Here, however, the close-range limit is very high (often around 20 meters). Such glasses are therefore mainly recommended for landscape exploration, at sea or e.g. for the observation of big game. Terms such as “Sports-Auto-Focus” (Steiner) or simply “Auto-Focus” are often used as synonyms for the fix focus. However, these terms are misleading, since this system has nothing to do with autofocus as known from cameras etc. (distance measurement or the like and motorized focusing).
Another focus variant is the separate focusing of both eyepieces. This system is mainly used for cheaper glasses for “harsh conditions”, since water and dust tightness is very easy to achieve.
Prism systems of the binoculars
The most frequently used prism systems are the more modern roof edge and the classic Porro prism system.
Porro binoculars (named after the Italian inventor Ignazio Porro) can be recognized at first glance by the fact that the lenses are much further apart than the eyepieces. In addition to this disadvantage in terms of compactness, there is also a small advantage in the imaging performance: The larger distance between the lenses makes the image appear more stereoscopic (better 3D representation).
Lenses and eyepieces form a line in roof prism binoculars. This system is more complex to construct, which usually also means a higher price. However, the result is a much more compact design. Only under the roof edge glasses are there those with real inner focus.
Inner focusing for Porro prism binoculars
the eyepieces are connected to each other by a bridge that can be moved back and forth by turning the focusing wheel. However, this eyepiece bridge is very fragile. At the slightest bend, the distances between the two eyepieces and the objectives no longer match. The result is two differently sharp images.
Most roof prism glasses, however, have an inner focus. The focus (focusing) is done by moving lenses inside the device. There are no moving parts on the outside of the glass. The mechanics are thus protected. Only with this system can real waterproofness be achieved without any problems.
Best and easiest to implement with roof prism binoculars. Devices with a Porro system are much more complex. Options include sealing with O-rings and filling with nitrogen to prevent the optical elements from fogging up inside the device. The whole works so well with the corresponding effort that you can e.g. can even go swimming with a Leica glass!
Binoculars – types of glass, finishing and surface coating
Optical systems refract light of different colors (wavelengths) to different extents. If you only used simple window glass for the construction of binoculars, the result would be an image with an unsightly variety of color borders.
This phenomenon can be almost completely eliminated by using special types of glass and coatings. Without this processing, the image quality, especially in the case of roof edge glass (which makes this system again more complex and expensive) would leave a lot to be desired.
Another important refinement of the optical glasses is the anti-reflective coating on the outer surfaces of the lenses and the coating of all glass / air surfaces:
Conventional lenses and prisms have the property of reflecting part of the incident light. There is a loss of light and a reduction in the contrast due to stray light. By coating the lenses (vapor deposition of a reflection-reducing mineral layer), reflections are considerably reduced and the light transmission is increased. Fully coated binoculars allow approx. 60% more light into the beam path compared to non-coated glass. The light intensity is further increased by using special coatings (UV coating, multi-layer coating etc.). So modern glasses let in up to 95% of the incident light!…