Review: 12x50 Optolyth Alpin (Classic) vs. Zeiss Jena Dodecarem vs. Zeiss Jena Nobilem Spezial

A magnification of 12x is inside the crossover region between hand-held binoculars and tripod mounted instruments. When held in free-hand mode, the jitter becomes very well visible, and many experienced observers claim those 12x binoculars to be unable to show any more details than 10x binoculars, unless they are mounted. I agree, in principle, but in real life there is still room for a compromise: Whenever the arms are partially supported, while resting on a windowsill or on the arm rests of a chair, 12x binoculars are still usable without tripod. For astro observations, one may lay back on a camp-bed and rest the binocular on ones face.

When Zeiss Jena introduced the 12x50 Nobilem Spezial (Nobilem-S) around 1980, and the Dodecarem around 1985, these instruments represented the high end class 12x binoculars of that time. Quite a lot has changed since then, in particular when considering the weather sealing and weight reduction of modern binoculars. The Optolyth Alpin (Classic) is such a modern representative of the current medium quality range and costs about the same as the Zeiss Jena glasses cost today when purchased second hand.

Fig. 1: The Optolyth Alpin (Classic) (current production)

The Optolyth Alpin is a series of low-weight Porro binoculars, ranging from 8x30 to 12x50. Recently, a variation, called 'Classic', has been introduced, which comes with a rubber armor of typical Zeiss-Classic look. The line includes a 8x40, 10x40, 7x42, 10x50 and 12x50. They are well made and their weight is amazingly low, although the apparent fields of view of the 8x40, 7x42 and 10x50 are quite narrow, indicating the implementation of rather small prisms. I have measured the exit pupil of the 12x50 and found just 3.5mm instead of the expected 4.2mm. As a consequence, this binocular has effectively got just 3.5mm x 12 = 42mm of un-obstructed entrance pupils, making it a 12x42 rather than 12x50. The generous eye-relief allows for a comfortable observation when eye-glasses are required, after the rubber eye-cups are folded down. According to their web-page, the prisms are shock-proof mounted, but the Alpin series is not waterproof. With a price of roughly 500 Euro, the Optolyth Alpin is a representative of the medium quality class, and a 30 year warranty is part of the package.

Fig. 2: The Zeiss Jena Nobilem Spezial (1981)

During the late 1970s, Zeiss Jena developed a new line of high quality binoculars, the 8x50 Nobilem Super and the 12x50 Nobilem Spezial. The goal was to create binoculars of wide fields of view and nevertheless compact design. This was achieved with the help of tele-objectives, doublets featuring a wide air space. Here, the distance between objective and image plane is less than the focal length, creating the short and stubby shape of the Nobilem-S. The ocular is a wide angle design made of 5 lens elements. Since the prisms are of extreme size, the body is correspondingly wide and it is difficult to focus this binocular with small hands, particularly the 12x version, when one hand has to move up to reach out for the focuser. The eye-relief is fairly long, but since the eye-lens is somewhat recessed, it is not sufficient for a spectacle wearer to see over the entire field of view. Apparently, the rubber eye-cups do not fold down (they are snapping back). These binoculars are not waterproof. Because of its sophisticated optical design, with low tolerances in production, the Nobilem-S was expensive, did not sell well, and was discontinued after just a few years. Today, collectors are frequently boosting its price up beyond 600 Euro.

Fig. 3: The Zeiss Jena Dodecarem (1987). The original price-tag asks for 1170 Mark plus 90 Mark (for the leather case)

Around 1985 Zeiss Jena replaced the Nobilem-S with another, more conventional construction. Initially, these were called '8x50 Octarem' and '12x50 Dodecarem', but later on both were renamed to 'Nobilem'. By 1990, a 7x50, 8x56, 10x50 and 15x60 had been added to complete the Nobilem line. My Dodecarem arrived in original box with price tag, indicating a price of 1170 (East German) Mark, plus 90 Mark for the leather case. This was probably close to the average monthly income of an East German worker at that time. The Dodecarem has got a somewhat reduced, but still wide field of view, conventional doublet objectives without air space and simpler 4-element oculars. The body is long compared to the Nobilem-S, with a characteristically curved prism-housing cover. The focuser is located at the center of the hinge, much more convenient to reach. The eye-relief is somewhat short, but the rubber eye-cups fold down so that eye-glass wearers should be able to see almost over the entire field of view. The Dodecarem is splash-waterproof and comes with cemented prisms and shock-proof mounting. Second hand, it sells nowadays for typically 400 Euro. Today, Docter Optics is still making the 7x50, 8x56, 10x50 and 15x60 Nobilem.

Fig. 4: The Nobilem-S, Alpin (Classic) and Dodecarem. One may compare the different size of their prism-housing.

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

Optical performance

Image sharpness: Is perfect within the central region of the image. About 70% towards the edge, the Nobilem-S displays the first traces of image blur, and the same happens with the Alpin and Dodecarem at roughly 80%. When comparing these results with those of other binoculars of similarly wide apparent fields, obviously all three of them perform rather well. In fact, with 12x magnification, the true field of view becomes rather narrow, which helps to reduce the amount of certain aberrations that grow steeply with the angle of field.

Image color: The Alpin comes closest to 'neutral' - I would regard its image tint being shifted a little towards 'warm'. The Dodecarem shows a slight yellowish color, which is more pronounced through the Nobilem-S. After having seen several Nobilem binoculars, I suspect that most of the older samples show a yellowish tint, although its intensity may vary a little among different samples. Perhaps what we observe here is an effect of ageing, and the particular history of each individual sample may have a certain influence on its color rendition.

Rectilinear distortion: All three contenders display a small amount of pincushion distortion, which is implemented intentionally to compensate for the globe effect while panning.

Stray light: In fact, the amount of diffuse stray light is rather low in all three samples. Both Nobilem-S and Dodecarem show some traces of stray light after sunset, when observing a shadowed area, with the residual sky-glow in background. During daytime and under normal light no such effects show up. Here, the Dodecarem is clearly superior to its 8x50 brother, the Octarem (later called 8x50 Nobilem), which usually displays a faint diffuse ring of stray light towards the outer region of its image. When taking the formula for the intermediate image size (above in paragraph 'Angle of view'), and assuming Fr = 4, we obtain S = 28mm for the Octarem and S = 20mm for the Dodecarem. The 12x50 has got a smaller intermediate image, so that perhaps internal stray light baffles have been made rather narrow, which helps to prevent stray light from entering the light path. In contrast, the 8x50, with much larger exit pupil d, has to accommodate a wider light cone and hence stray light protection becomes more difficult. The Optolyth Alpin displays an excellent stray light protection. This could well be a positive side effect of its stopped down entrance pupil: Light rays entering near the edge of the objective lens are banned, and similarly those which would hit the internal tube walls and become reflected back through flat angles. An effective stray light protection requires space - and the Alpin, being a 12x42 inside 12x50 tubes, has got this extra empty space which is able to absorb a good amount of nasty reflections.

Ghost images: The ghosts are produced when light of a bright source is reflected at one of the air-to-glass surfaces and, after a second reflection, scattered back into the light path. A high quality anti-reflection coating helps to diminish these ghosting effects. When checking with the moon or a usual street lantern in the night, both Alpin and Dodecarem are producing a few ghost images of low-to-moderate intensity. It is interesting to note that the Nobilem-S appears to be less prone to ghosting, which is surprising since both Nobilem-S and Dodecarem should have the same type and quality of coating. The puzzle is solved when observing a light source of very high intensity (the flood-lights of a distant soccer stadium): The Nobilem-S produces a number of ghosts similar to the Dodecarem, but most of them are rather de-localized, hence of less intensity, and they remain invisible with light sources of 'normal' intensity. This is an example where the particular ocular construction shows up as an additional variable for ghost-prevention. When considering the fact that the Optolyth Alpin was produced more than 20 years after the Zeiss Jena glasses, it becomes clear that its so called 'Ceralin-plus' coating is of just average performance. I have seen more effective coatings on current medium quality binoculars, the Nikon EII being one example.

Low light performance: Here, the Dodecarem is a clear winner. Already during daylight, its image appears bright compared to the Nobilem-S. The latter has got a few additional air-to-glass surfaces (air-spaced objectives, prisms not cemented, one additional ocular lens) and, with its oversized prisms, a lot of glass to be passed. This seems to create a visible reduction of transmission. The Alpin is a special case: During daytime, its image brightness is superior to the Dodecarem's. Under low light conditions, however, its narrow exit pupil is turning into a major handicap, and here its performance quickly drops below both competitors. This is no surprise: Under dim light, once the eye pupils have grown beyond 4.2mm, the (true) 50mm binoculars collect about 40% more light than the (effective) 42mm of the Alpin.

Mechanical construction

Summary

The 'final score' is the sum of the individual scores and is intended to serve as an orientation only.

The Optolyth Alpin is a modern, medium quality binocular and its performance is, altogether, adequate for the price. This glass is very light and compact, has got a reasonable, although not wide, field of view with excellent stray light protection. Image brightness (during daytime) and edge sharpness are both beyond average. Mechanically it provides the feel of a quality product, although the lack of water resistance is limiting its spectrum of application. Its coating, however, appears to be less than state of the art within the current medium quality class, and hence this binocular is somewhat prone to ghosting. In fact, this coating appears to be on a level similar to the Zeiss Jena coatings 20 years ago. The only significant drawback is its poor performance under low light: With an exit pupil of just 3.5mm, the Alpin performs like a 12x42 binocular and this may not be sufficient for a birder who is planning to observe animals under shady trees. I urge Optolyth to be more explicit here: This is a 12x42 binocular and it should be specified accordingly. This instrument should either be equipped with 42mm lenses, or its field of view should be reduced to accommodate the entire light cone and deliver the entire 4.2mm exit pupil. Preferably, of course, prisms of adequate size should be installed, which would allow for both a reasonably wide field and a complete exit pupil. With 200g extra weight, this glass were still fairly light and compact, and such an investment into prism size would deliver another performance boost to this binocular and to the entire Alpin series.

The Zeiss Jena binoculars were representing the high end 20-25 years ago and are still fully competitive with today's medium quality range. The Dodecarem was the successor of the Nobilem Spezial and has the advantage of improved transmission, edge sharpness and mechanical ruggedness. In turn, the Nobilem-S has got the wider field and is marginally less prone to ghosting. Both exhibit a yellowish color rendition, whose intensity may differ individually and in this case it was stronger with the Nobilem-S. When comparing the 12x50 Dodecarem with the 8x50 Octarem, it is interesting to observe how the 12x50 displays a significantly lower level of diffuse stray light. As mentioned above, the intermediate image is smaller with the Dodecarem, and hence the stray light stops may have been designed more tightly compared to the Octarem which has to pass a rather wide light cone. For the same reason, the prisms are somewhat oversized in both 12x50 models: They were designed to accommodate the wider light cones of the 8x50 models of the same production line. Therefore, the 12x50 models could have been, in principle, designed with more compact bodies, but this would have led to a further increase of production costs, which were already on the high side.

Today, manufacturers usually decide for the opposite direction: In order to minimize weight and bulk, prisms are chosen as small as possible, and consequently, the appearance of tiny angles of field are often tolerated. These narrow fields also permit the designer to chose a simple ocular construction instead of a sophisticated wide angle design, which helps to further reduce costs. In the extreme case, the excessive trimming-down of prism size is causing an effective reduction of the entrance pupil and seriously affects the binocular's low light performance, as happened to the Optolyth Alpin.

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