Nikon 7x50 and 10x50 WX binoculars


by Holger Merlitz



In April 2017, Nikon has announced the new Nikon WX Super-wide field binoculars for use in astronomy. These binoculars are remarkable, because they are squarely opposing common trends in high-end binocular design, which generally favor compact, lightweight and easy to handle all-purpose binoculars. The following table contains a subset of specifications, which clearly demonstrate the unique mix of properties of these devices:


  Real angle Apparent angle Apparent angle Eye relief Close focus Weight
  of view (deg) ISO-Norm (*) angle-condition (**) (mm) (m) (kg)
Nikon 7x50 WX 10.7 66.6 74.9 17.7 12.3 2.42
Nikon 10x50 WX 9 76.4 90 15.3 20 2.51
(*) The ISO Norm 14132-1:2001 assumes absence of distortion

(**) The angle condition is the product of real angle and magnification



The new binoculars are equipped with huge angles of view, exceeding everything offered before on similar instruments. Nikon specifies the apparent (subjective) angles following the ISO Norm 14132-1:2001. This norm assumes the complete absence of distortion and yields a purely theoretical value. Similarly, the angle condition, being the most common approach to estimate subjective angles, assumes the presence of a considerable amount of pincushion distortion. In reality, subjective angles are measured somewhere in between both limits. I have inspected both binoculars on the IWA in Nuremberg and observed rather low levels of pincushion distortion in their images, so that the ISO specification is certainly closer to the truth than the angle condition. I would urge Nikon to publish the precise (laboratory) values here, because computed specifications are often so vastly different at these wide angles.

There exist further unusual properties of these instruments: They are focused individually at their eyepieces, and their close focus distances are comparably huge - both features in fact suggesting applications primarily in the field of astronomy, although any long distance terrestrial observations, for example from elevated scenic outposts, should yield outstanding results, too. The considerable weight of this binocular of about 2.5 kg suggests it to be mounted on a tripod. The instrument is sufficiently compact that handheld observations seem possible whenever the ellbows find a suitable support. We shall now take a look inside and see what these monsters have to offer:

Visible to the left is an air-spaced doublet objective, followed by an Abbe-Koenig prism. At its exit, a massive block is located, composed of two negative lenses with air-space. This is the field-flattener system. The remaining part of the eyepiece seems to be a design composed of four groups in a 1-1-1-2 array. According to Nikon, these eyepieces are derived from their NAV-HW line of wide-angle astro-oculars. Naturally, the 7x50 and 10x50 are going to feature different eyepieces, and I am not sure whether this sketch actually shows the former or the latter. According to Nikon, three ED (extra low dispersion) glass elements are used in each barrel. Altogether, this is quite an impressive setup, and it may be submerged under water and remain waterproof up to 10 minutes at a depth of 5m (Nikon laboratory specification). We shall now address a couple of questions which may arise when discussing these unusual binoculars.

Why is it so heavy?

Let us first estimate the diameter of its intermediate (aerial) image. It is calculated as Z = 2*F*tan(A/2), with A being the real (objective) angle of view, and F the focal length. We do not know the value of F, but considering the sketch above, the focal ratio of the objective does not appear too fast, perhaps close to 1:4.5, which would yield F = 4.5*50mm = 225mm, and thus Z = 42mm. An aerial image diameter of 42mm indicates that the ray fan, after entering the objective, is hardly convergent. We may then safely assume that the entrance width, w, of the prism is of the same order as Z. The volume of an ordinary Abbe-Koenig prism is computed as V = 3.72 w^3 (see e.g. Paul R. Yoder, Jr., Daniel Vukobratovich, Field Guide to Binoculars and Scopes, SPIE 2011). We do not know the specific weight of the optical glass used for the prism. At least the second element of the cluster may be made of low index glass (BK7 has a specific weight of 2.5 g/cm^3), while the first block may consist of high index BaK4 (3.1 g/cm^3). We take a middle ground and assume 2.8 g/cm^3, to obtain a weight of 772g. The figure to the left indicates that Nikon has not spared efforts to cut away all edges from the glass block that are not needed, and thus may have saved another 25% of the total weight. We would then end up with a weight of roughly 580g per prism cluster. The cutaway image indicates that the eyepieces, including field-flattener, contain a lot of glass, too, and together with the objectives we may add another 400g to each barrel, summing all glass elements up to something close to 2kg in both barrels. It is then no surprise to read that the entire instrument weights 2.5kg. This applies to the 7x50 as well as the 10x50, because both certainly use identical prism clusters.

Why Abbe Koenig prisms?

We could repeat the calculation of the previous section, assuming Schmidt Pechan (SP) prisms, which are made of high index glass and have a volume of V = 1.8 w^3, arriving at a weight of 413g per prism cluster. Perhaps, a little bit of glass may be removable, so that a cluster would weight about 350g. The entire binocular could be lighter by roughly one pound, if it were using Schmidt Pechan prisms. Why has Nikon decided to use Abbe Koenig (AK) instead? I have been told that the motivation behind that choice was the reflective layer that is needed for the SP system, in contrast to the AK in which all reflections are based on total internal reflection and thus without any loss. This argument is hardly convincing, since high quality dielectric mirrors reach reflectivities of the order of 99.8% and thus render any losses insignificant. I rather believe that it is a fundamental difficulty with the SP setup, which made them choose the optically superior, but much heavier AK system: As demonstrated by Swarovski's optical designer Konrad Seil in 1991, the SP system is losing contrast as a result of its antireflection coating (pp. 55-56). A fully multi-coated SP prism has an MTF value which is, at a spacial frequency of 25 lp/mm, easily half as high as a prism with single layer coating. Therefore, the SP prism sacrifices either light, or contrast, and this fact is entirely unrelated to its mirror layer. It is a consequence of the fact that the SP prism uses one and the same surface as entrance resp. exit surface, as well as for total internal reflection. The coating supports the former, but has a damaging effect on the latter. Nikon could have selected Porro prisms, which are simpler technically and at least as good optically as the AK system, but a slim design was preferred.

Why no central focusing?

The cutaway image suggests that there is some free space left between objective lens and prism entrance. Why then has Nikon not implemented an internal focusing lens? This is a matter of speculation. Fact is that a focusing lens may have a negative impact on the image quality: The cluster, consisting of objective lens plus focusing lens, may be perfectly corrected only for a single particular position of the latter. Once the lens is moved away from its optimum position, while the user adjusts for his individual eyesight, a residual chromatic aberration or spherical aberration may arise and reduce the image quality. Alternately, the eyepieces may be displaced to achieve a focus without any image degradation. But that would be hard to seal, in particular since the eyepieces are of such a massive size that the corresponding o-rings would generate a considerable amount of friction. It is therefore understandable why Nikon decided to use the simple, but reliable solution of individual eyepiece focusing.

Summary: What have we got here?

The picture to the left shows Dr. Hans Seeger during his inspection of the Nikon 7x50 WX (March 2017 on the IWA). Both, the 7x50 and 10x50 offer images that are superior to anything I have seen so far in any binocular. The images are not only very wide, but amazingly bright, of highest contrast, almost sharp to the edges, and impress with their almost perfect correction of chromatic aberration. Of course, the conditions inside the exhibition hall have been far from being perfect for the conduction of critical tests, and I would be eager to continue my tests under realistic conditions in the field. One or the other weakness may show up, but I can hardly imagine that my first impression would fail me in this case: These are the best binoculars ever produced, and as such they are, at least to me, the most exciting introduction to the binocular market of the century.

However, the prices of these devices are reaching a level of 6000 US$, and are certainly prohibitive to almost every amateur astronomer, even the most enthusiastic one. Another drawback is the weight of 2.5 kg, which is somewhat beyond the limit of comfortably handheld instruments. I would love to see extensions of the current WX line toward smaller devices. For example, a 7x35 with identical objective angle of 10.7 deg. and the same focal ratio as the 7x50 would yield a diameter of the intermediate image of about Z = 29.4 mm, and prisms that weight only 1/3 of the currently employed prism. Similarly, the diameter of the eyepiece would scale down, since its huge field lens would reduce considerably in size. The resulting 7x35 wide-angle instrument would possibly weight about 1.2 kg, were easily handheld and carried around.

Disclaimer

The information given in this report reflects the personal impression and opinion of the author only. I cannot guarantee for the accuracy of any given specification. I have neither been payed nor have I been supported in any other way to write this review.

Back Home

Last modified: April 2017