Review: 10x50 Swift Kestrel vs. Zeiss Jenoptem vs. Hensoldt Diagon

In another review, two high-end 10x50 binoculars (Fujinon FMTR-SX and Docter Nobilem) were compared and tested against the Jenoptem. These were glasses of the 700 Euro range, representing the finest Porro-type binoculars presently available in the 10x50 class. Although roof-prisms of much higher price can be found, e.g. at Leica and Swarovski, these are simply more compact and of less weight, but not significantly better in their optical performance.

This review is focusing on three wide-angle 10x50 binoculars of the middle class, i.e. the price region of 200-250 Euro. These devices, although not waterproof, offer a decent optical performance combined with a mechanical construction which is noticeably more reliable and homogeneous than usually found in the cheaper models for 100 Euro or below. The Zeiss Jenoptem and the Hensoldt Diagon are classical binoculars, available since several decades and not in production any more, but available on the second hand markets.

Fig.1: The Swift Kestrel 10x50 (Audubon 826).

While 'Swift Instruments, Inc.' is a US trademark, the Kestrel (also called 'Audubon No. 826') is actually made in Japan. There also exists the 8.5x44 version (Audubon No. 804) which has apparently got an excellent reputation in the bird watching community. The 10x50 is claimed to be a binocular for birding and astronomy - not that these applications appear that much similar. According to the brochure, the Audubons come with a one piece metal body and a patented 'Dura-Shock' prism seat for enduring quality. The rubber eye-cups can be down-folded for eye-glass wearer, and the 15mm eye-relief should be just enough to be used with spectacles on. A nice feature is the rubber protection around the objectives - it allows to rest the binocular on a hard surface without the danger of scratching either the surface or the instrument. Another plus is the focusing wheel which is conveniently located in the center of the bridge, allowing to focus the device while it is firmly held in both hands and optimum balance. The MRP of the Kestrel was much beyond $300 US, but it is actually found in many places for $220-$250.

Fig. 2: The Zeiss Jena Jenoptem 10x50W. The rubber eye-caps were not standard.

Whether it was called 'Dekarem', 'Jenoptem', 'Aus Jena', or, after 1991, 'Docter Classic', it stood for more or less the same product. According to Docter, this binocular was, in the present form, computed already in the late 1940s (as were the 8x30 and 7x50), and its optical formula remained unchanged over half a century. However, the surface coatings were improved a few times, and from 1978 onwards all models were equipped with multilayer coatings. Also, countless constructional changes were implemented over the years, to reduce the number of parts and improve the efficiency of assembly. Different names were dictated by the importers only, and the same instrument was usually produced with various different name plates. The 10x50 is identical to the 7x50 except the oculars. The Jenoptem 10x50W to be tested has a multilayer coating and is equipped with a central focuser. It can be frequently found on E-bay for about 200 Euro in good condition.

Fig. 3: The Hensoldt Diagon 10x50

The Diagon was the civilian version of the famous Hensoldt DF 10x50, delivered to the Bundeswehr from 1955 to the early 1970s. In contrast to its military counterpart, the Diagon came with central focuser, was not waterproof and without the reticle and rubber armor. Apparently it wasn't sold for a long time, and nowadays it is not so often found on the surplus market. This seems understandable: In 1954, Carl Zeiss Oberkochen had acquired a majority stock holding in Hensoldt and by 1957 Zeiss introduced its own superb 10x50 binocular, and it is obvious that they did not want the Diagon to be around any longer as an in-house competition to its own products. It is therefore likely that the Diagon to be tested was already made in the 1950s (serial no. 863700), and consequently its coating is far from being up on today's standards. In good condition, this binocular sells for 250 Euro, the military version often goes beyond 300 Euro.

Fig. 4: The Jenoptem, Kestrel and Diagon

The following table summarizes some of the specifications of the contenders.

Optical performance

Image sharpness: Not surprisingly, the sharpness and resolution of these instruments is excellent in the central part of the field. All three of them show nice point-like stars close to the center, with a slowly increasing distortion toward the edge of the field. In the Jenoptem, the aberrations become dominant about 60% (radially) away from the center, and close to the edge the stars are grossly deformed. The Diagon and Kestrel appear somewhat better, the stars are tolerable up to 70% of the field, but again, close to the edge, the distortions have reached a high level. None of these binoculars is reaching the outer-field sharpness of the high-end devices like Fujinon or Nobilem.

Image color: All three of the glasses provide a nice neutral image color. I can't see differences of any relevance.

Rectilinear distortion: All three of them show a slight pincushion distortion to eliminate the globe effect while panning. The panning behavior of these binoculars is quite similar and with excellent smooth characteristics. The Hensoldt had since long gained a particular reputation for its smooth panning, but neither the Kestrel nor the Jenoptem are prone to produce any disturbing effects as well.

Stray light: Light from sources outside the field of view may enter the instrument through large angles and illuminate the internal tube walls, is in parts scattered back into the optical path and produces a diffuse illumination of the image. Here the Kestrel exhibits some problems: Not only under difficult light conditions, but also when used under daylight, this binocular seems to be prone to suffer from loss of contrast due to stray light. As usual, this effect becomes more pronounced under twilight conditions, when residual glow of the sky is disturbing the observation of scarcely illuminated areas on the ground. A closer look into the objective tubes reveals that the side-wall of the prism (which leads to the ocular) is positioned dangerously close to the optical path, too close to be shielded by the baffle, and not even painted black. This could be one cause of unwanted reflections. The Jenoptem is suffering significantly less from stray light, and even better performs the Diagon. The latter has almost got the excellent stray light suppression of East European military binoculars. This is achieved by a single, long 'baffle' which starts at the objective lens and extents half way toward the prism-entrance, like a 'tube inside the tube' with an increased slope. The same strategy is also applied in the Hensoldt DF 8x30 and the NVA DF 7x40. Quite efficient, but it requires wide tubes and may add to the weight of the binocular.

Ghost images: If, at night, a bright object (street lantern, moon) is positioned into the field, reflections on the air-to-glass surfaces take place, which can lead to multiple 'ghost' images of the light source. The Kestrel shows one distance-dependent effect: When the glass is positioned at normal distance from the eyes, a few dim ghost images are present, which almost completely disappear when the instrument is shifted closer to the eyes (for that, the rubber eye-cups have to be folded). The Jenoptem displays one fairly bright spot and a couple of dim and diffuse images of the light source. The Diagon, however, is suffering badly, with several bright to very bright secondary images, which are quite disturbing when the moon is observed. Generally, the resistance to ghosting is an indicator for the quality of the coating, which is high in the Swift and fairly high in the Zeiss. It is not surprising that the 50 years old Hensoldt is not competitive in this discipline. The intensity of some of the ghosts, however, makes me suspect whether Hensoldt might have decided (for the civilian Diagon, perhaps not for the military version) to save expenses and to leave some of the lenses uncoated.

Low light performance: Is closely related to the image brightness, and, to some extent, also influenced by the color (tint) of the image. The brightness depends on several factors, as there are the number and thickness of optical elements, the number of air-to-glass transits (which could be reduced e.g. by cementing together the prisms) and the quality of the surface coatings. Last not least, factors reducing the contrast, like stray light, could affect the low light performance of the instrument. The latter is the case with the Swift under twilight conditions, but with decreasing residual light this effect disappears, and now, thanks to the fine quality of its coating, the Kestrel is performing well. The Jenoptem, with its multi-coated lenses and prisms, is never visibly behind the Japanese glass, proving it to be still competitive with modern binoculars. With its excellent stray light suppression, the Hensoldt is on par with its competitors as long as a fair amount of residual light is there, but once it is getting darker, it is clearly falling behind. Here the light transmission-rate is significantly reduced, with a surface coating far from being state of the art, and this affects its performance during low light terrestrial and astronomical observations as well.

Mechanical construction

Summary

The 'final score' is the sum of the individual scores and is intended to serve as an orientation only. Generally, it would be an over-simplification of the matter to just look which binocular has got the highest score, because it would obscure the individual features of the devices which differ quite a lot among each other.

The Swift Kestrel is an excellent representative for the middle class 10x50 binoculars. It is altogether a bit ahead of the two contenders, which, although not in production any more, defined the upper class in the 1950s. A big plus for the Kestrel is its compactness, low weight and excellent handling. It is not without deficiencies, however. Its tendency to suffer from stray light, especially under twilight conditions, may be the price for its compact size, but the fact that the side-walls of the prisms are not painted is simply a flaw. Its edge-sharpness is all right for a wide angle binocular, but not as good as it could be. Altogether, it does not reach the performance level of the Fujinon FMTR-SX and the Docter Nobilem, but it may be regarded as a legitimate, in optical performance and handling improved successor of the Zeiss Jenoptem/Dekarem, and it comes with a reasonable price-tag.

The Zeiss Jenoptem has proved to be still competitive among the middle class 10x50 binoculars. A characteristic feature is its wide angle of view. Although the image sharpness is quickly going down toward the edge, both stray light and ghost image suppression are reasonably good, and the versions with multi-coating (these are all Zeiss Jena glasses produced 1978 or later, i.e. serial numbers beyond about 4800000) also deliver a decent low light performance. With just above 1 kg of weight it is pretty mobile, but, like the other two contenders, it should not be excessively exposed to water or dust. There have been rumors about the late Jenoptems being mechanically inferior to the older Dekarems. Most likely this was caused by 'faked' or 'oriental' copies of the Jenoptem, which were produced in Japan during the 1980s and allegedly had a sloppy mechanical construction.

The Hensoldt Diagon was a great binocular in the 1950s and 1960s, but its surface coating is not competitive any more by today's standards. Still, the edge sharpness is fairly good, the stray light suppression is excellent (perhaps a heritage of its military origin) and, despite of its age of half a century, it has conserved a neutral image color. One question was frequently posed: Which of the classical rivals, the East German Zeiss Dekarem/Jenoptem or the West German Hensoldt DF/Diagon was the better binocular? The answer is a little involved, because it requires the inclusion of the time of production. In the 1950s, no doubt the Hensoldt had the advantage over the Dekarem, with a better edge sharpness and stray light suppression, and with a ghost image suppression and low light features which were that time probably on par with the glass made in Jena. The late versions of the Dekarem and the Jenoptem, however, had got a slight advantage due to their multi-coated lenses (the Hensoldt was not in production any more in the late 1970s). Today, Hensoldt delivers the new Fero-D 19 to the Bundeswehr, which has got a smaller field of view (6.3 degs.), is nitrogen filled and comes with a state of the art coating and laser protection filters.

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