by Holger Merlitz
In April 2017, Nikon has
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:
||of view (deg)
|Nikon 7x50 WX
|Nikon 10x50 WX
(*) 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
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.
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.
Last modified: April 2017