Review: 7x50 Miyauchi Binon vs. Fujinon FMT-SX vs. Tasco No. 124

by Holger Merlitz

Binoculars of size 7x50 are traditionally dominant in naval applications. There are a couple of reasons for that: First of all, magnifications higher than 7x were difficult to use once the observer is standing on moving ground, i.e. on a rolling ship where it is almost impossible to firmly fix an object with a 10x binocular. Then, a good low-light performance is wanted for use under difficult light conditions. Finally, although 7x50 binoculars are rather heavy, this disadvantage is of little relevance on a boat where there is no need to carry it around over any longer distance.

Under dark skies, these binoculars could as well be excellent candidates for astronomical applications, but they are rarely found there, mostly because of their small fields of view: As was already discussed elsewhere, it is difficult to construct binoculars with both large exit pupils and wide angles, because these conditions, if to be realized simultaneously, require the use of low F-ratios as well as oversized oculars and prisms. It is therefore no surprise to find a typical 7x50 binocular with only 50 degs. apparent field of view (AFOV), and a combination of 60 degs. AFOV and 7mm exit pupil is quite rare (one example is the Zeiss Victory 8x56).

Recently, Miyauchi, well known as a manufacturer of high quality and large-sized binoculars for astronomy, has taken over the challenge to create a new 7x50 wide angle binocular with impressive 66.5 degs. AFOV, filling a gap which had become increasingly wide after the production of the Celestron Nova 7x50 and the Swift Commodore Mark II had ceased in the 1980s. In this review, the Miyauchi Binon is compared to an older wide angle binocular, the Tasco No. 124, and one of today's high-end representatives of conventional 7x50 binoculars, the Fujinon 7x50 FMT-SX.

Fig. 1: The Miyauchi 7x50W Binon

The Miyauchi Binon comes with a couple of spectacular features: It is of Porro II construction, thereby reviving a prism type which was successfully applied in the famous Carl Zeiss 8x60 of the WWII era and which later on almost disappeared from the market of hand held binoculars, with exception of a few British products by Ross and Barr & Stroud. Its objective lenses are triplets, indicating Miyauchi's serious attempt to fight the aberrations inherent in any telescope with a focal ratio as low as 3.2. The individually focusing oculars are of 5-lens Erfle type with an excellent eye-relief of 22mm. Unusual is their feature of turning in opposite directions, i.e. to focus a close object, the left ocular has to be turned clockwise and the right one anti-clockwise. The diopter settings range from about +5.5 to -8, and the close focusing distance is specified as 5m. The entire construction is fully waterproof and nitrogen filled, making the Binon usable under any weather conditions. With 1.25 kg, it is also comparably light for a 7x50 wide angle. When summing up all these features, the price of about 600 Euro asked for the Miyauchi Binon appears to be adequate.

Fig. 2: The Fujinon 7x50 FMT-SX

The Fujinon 7x50 FMT-SX represents today's top line of 7x50 binoculars, along with the Zeiss 7x50 B/GA, the Nikon Prostar and, perhaps, the Docter Nobilem 7x50. Like the 10x50 FMTR-SX and the 8x30 FMTR-SX, it complies to US military specifications regarding ruggedness and resistance against pressurized water. The coating (indicated with 'SX') is of highest quality available these days. Its ocular construction (with 5 lens elements, individual focusing, diopter setting from +6 to -6) contains a field-flattening lens to reduce the field-curvature, improve edge-sharpness and eye-relief, which is with 23mm as generous as the Miyauchi's. Of cause, the Fujinon is of conventional design, i.e. its true field of view (TFOV) is, with 7.5 degs., only slightly above the standard of 7.1 degs. Also, its weight is, with 1.45 kg, on the high side and not far from the limit of what can be comfortably held for a few minutes without external support. The Fujinon 7x50 FMT-SX can be found for around 500 $US, which I regard to be highly competitive - its most serious competitors cost 1000 $US (Zeiss B/GA) and 700 $US (Nikon Prostar).

Fig. 3: The Tasco 7x50 No. 124

The Tasco 7x50 No. 124 is one representative of a remarkable line of wide angle binoculars, introduced by Japanese manufacturers during the 1960s and sold under various brand names until the 1980s, when customers asked for more compact and stylish roof prism types, thereby leading the 'golden era of wide angle binoculars' to an end. The Tasco 124 came with amazing 11 degs. TFOV (which corresponds to 77 degs. AFOV), was nevertheless quite compact and, with 1.2 kg, of surprisingly low weight. This was achieved by using magnesium as material for the body - nowadays still found in a couple of high end binoculars. Another advantage is its central focuser, making it easy to hold the 1.2 kg firmly in both hands while focusing. This glass is not waterproof, however, and the eye-relief of 13mm is a little short to be of use with spectacles. The Tasco 124 and similar wide angle binoculars are still found on the surplus markets, e.g. on US Ebay, and then sell for typically 100-150 $US, depending on condition.

Fig. 4: The Miyauchi Binon, Fujinon FMT-SX and Tasco No. 124

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


  Real angle Apparent angle Eye relief Exit pupil Weight
  of view (deg) of view (deg) (mm) diam. (mm) (kg)
Fujinon 7x50 FMT-SX 7.5 52.5 23 7.1 1.45
Miyauchi 7x50W Binon 9.5 66.5 22 7.1 1.25
Tasco 7x50 No. 124 11.0 77 13 7.1 1.2



Optical performance

Angle of view: The official specifications appear to be correct. The Tasco has got a most impressive wide field of view, which is useful for observations under daylight, where the un-sharpness of the image in the outer fields remains less apparent. The Miyauchi's 9.5 degs. provide an excellent compromise. They avoid the tunnel-view, so typical for 7x50 binoculars, and are thereby making full use of the potential inherent in this kind of low-magnification binoculars: The potential to oversee a wide area for observation. This ability is missing in the Fujinon: With 7.5 degs, it provides little more than any wide angle 10x binocular.

Image sharpness: In the center of field, all candidates provide a properly sharp image. Big differences show up once the outer parts of the fields are compared. The Fujinon clearly performs best: In the star test, about 80% of the field (radially, starting from the center) show a star (of magnitude between 2 and 3) as a point, whereas further out the distortions are coming through, reaching a moderate level close to the edge. In the Miyauchi I have observed an asymmetric behavior, which could indicate a de-centering of the optics: If the stars are focused in the center of the field, then distortions become visible at 50% towards the upper edge of the field, and at about 70% towards the lower edge. The Miyauchi thus appears to belong to the 60% class with some de-centering so that the center of symmetry is positioned roughly 10% below the center of field. Amazingly, this is identically the case through both tubes. So far I was unable to receive any statement from Miyauchi, but one knowledgeable contributor to our discussion group suggested the following explanation: Suppose one of the tubes is not properly centered (i.e. the prism may be slightly tilted), then, during the collimation process, the second prism is set into alignment with the first prism and now produces exactly the same amount of de-centering. This seems to have no effect on the image quality in the central region of the field. Ironically, I even found one advantage of this 'feature': During daylight observations there are more interesting objects located in the lower part of the image, which is now of higher sharpness, whereas the uppermost area is often covering the sky, and here the un-sharpness is of no relevance. In astronomical observations, however, this asymmetry becomes obvious and is also disturbing. The Tasco is of limited use for astronomy: Slightly below 50% towards the edge, distortions become visible, and the outermost 20% of the field are not at all usable, showing the stars to be grossly deformed. In daylight use, however, the huge field of view is impressive and the un-sharpness far outside the center is tolerable.

Image color: All three binoculars display a neutral color rendition. The image brightness of both the Fujinon and the Miyauchi is significantly higher than the Tasco's, indicating their state of the art coating technology.

Rectilinear distortion: The Miyauchi as well as the Tasco have got the usual amount of pincushion distortion as it is commonly employed to compensate for the globe effect and to provide a smooth panning of the image. The Fujinon displays a less than average amount of distortion of the still image. A slight residual image-curvature is therefore apparent with the panning image (objects seem to come close towards the center and to move away towards the edge), but this is of very little impact because of the Fujinon's narrow field of view.

Stray light: A diffuse stray light is produced when bright objects outside the field of view illuminate the inner tube- or prism-walls and parts of this light is scattered into the light path, thereby causing a reduction of image contrast. The Fujinon is almost perfect here. Even under twilight conditions, when parts of the sky are still fairly bright, the observation of shadowed areas under the trees is possible without any loss of contrast. Here, the Fujinon 7x50 is clearly stronger than the Fujinon 10x50 FMTR-SX. The Miyauchi and Tasco show a slight and hardly significant amount of stray light and are as well suited for use under any difficult light conditions. All three binoculars can develop some reflections under very specific conditions, however: During night observations it is always possible to position a bright object in such a way outside the field of view that some of its light is reaching the exit pupil. It seems that this effect can be produced with any binocular, only the particular angle, under which the object is causing trouble, may differ.

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. A successful suppression of these ghosts indicates a high quality of the anti-reflection coating. The Fujinon and Miyauchi both belong to the best of the best. It requires light sources of high intensity to produce a dim, greenish spot, which remains localized and does not affect the contrast of the image. Observations of the moon are possible with hardly any disturbance by ghost images. The Tasco is producing a couple of ghosts of moderate-to-high intensity. It is interesting to observe how much the quality of coating has improved since those days of single layer coating as it was applied to the Tasco of the 1960s.

Low light performance: It is surely the most important feature of this type of binoculars. With an exit pupil of 7.1mm these are specialists for observations under twilight or even at night with small amounts of residual light. Again, the Fujinon and the Miyauchi are taking profit of their top quality coating. Already under daylight, their images show an impressive degree of brightness, and this is as well the case under dim residual light. If compared to the Tasco, their significant advantage in transmission becomes obvious. They show detailed structures of trees and their branches where the Tasco has already given up. I was unable to detect any difference in performance between the Miyauchi and the Fujinon. Both are state of the art low light binoculars.

Mechanical construction

The Fujinon complies to US military specifications with corresponding resistance against mechanical impact and pressurized water. The FMT-SX is, in contrast to the FMTR-SX, not rubberized and therefore not meant to be used under most challenging conditions, although the extra rubber skin is surely not making the difference in ruggedness but rather avoiding the odd look of scratched-off painting after excessive abuse. The focuser and the central hinge move only under significant application of force, with the advantage of staying where they are without the danger of being unintentionally changed. The same holds for the Miyauchi. Its construction appears to be extremely solid, although those parts covered with leather may not look as nice any more after extensive use under rugged conditions. I have no doubt that the manufacturer's claim of water resistance is correct, and I also believe that sea-water won't affect the function of this instrument. Again, the precious external appearance of the Miyauchi, if compared to the Fujinon's, may be somewhat more prone to suffer under excessive use in wet conditions. Its rubber eye-cups, as the Fujinon's, fold down to expose a long eye-relief for spectacle wearers. On first inspection one may doubt whether the Miyauchi can be held comfortably enough, because the oculars are heavy so that the center of mass of this glass is located slightly above the lower prism-house cover, and not further down at the beginning of the prism tubes. But this turns out to be irrelevant, since the instrument is small enough so that two fingers plus thumb hold the prism-house and the two remaining fingers comfortably grab the tube, providing a stable and firm hold. Focusing requires to hold the binocular with one hand, however, and this could turn out to be tedious with the Miyauchi and even more with the Fujinon. Here the Tasco with its central focuser is more convenient, although at 7x magnification focusing is not too difficult anyway. The Tasco has also the advantage of being rather compact. At low temperatures, however, its ocular lens tends to fog earlier than the Fujinon's or Miyauchi's - most likely because of its lower eye-relief which forces the eye to come closer to the cold lens-surface. As mentioned above, the Tasco is not waterproof, but otherwise of solid construction.

Summary

The following table is supposed to summarize the above observations. The best performing binocular gets three points, the following contenders two and one, respectively. In case several binoculars are ranking equally, their scores are averaged.


  Angle of Image Stray Ghost Low Image Mechanical Final
  field sharpness light image light color construction score
Fujinon FMT-SX 1 3 3 2.5 2.5 2 3 17
Miyauchi Binon 2 2 1.5 2.5 2.5 2 2 14.5
Tasco No. 124 3 1 1.5 1 1 2 1 10.5



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

The Fujinon FMT-SX is a technically perfect 7x50 binocular. It combines an excellent, flat image with a low level of edge distortion, a high-quality coating and a rock solid mechanical construction to form a state of the art military-grade naval binocular. There is nothing left to complain about, with two exceptions: Its field of view is narrow and its weight is too high.

These two shortcomings are properly addressed in the Miyauchi Binon. If the Fujinon is the most perfect 7x50 (perhaps challenged by the Zeiss B/GA or the Nikon Prostar), the Miyauchi is the most impressive one. What cannot be easily expressed in terms of numbers is the positive impact caused by its bright and wide field of view. Although its edge sharpness is not perfect, it comes with a similarly high level coating as the Fujinon, making it a perfect low light instrument for astronomy (under dark skies) as well as for hunting. Being waterproof, there are no limitations of its spectrum of application. As mentioned above, its surface appears to be somewhat more sensitive than the Fujinon's - that's the only reason why I won't suggest to abuse this instrument too frequently under combat-like conditions. The Binon I have tested displays a certain amount of de-centering. This is not to be confused with misalignment: Both tubes are perfectly parallel, but the center of sharpness is positioned about 1 deg. below the center of field, which could indicate that both prisms are tilted by one half deg. I assume that this has been overseen by Miyauchi's after-production control. Although this should not happen, one has to take into account that this glass is probably of the first production run (serial no. 00041) and that these initial problems will be sorted out very soon. Altogether I regard this binocular to be a successful experiment, and, being the only wide angle 7x50 on the market, it will find its happy owners (I am one of them). What comes next? Keeping the oculars and prisms untouched, one could construct an 8.5x60 or 9x63, both excellent wide angle binoculars for hand held use. A 10x70 would likely become heavy enough to ask for a tripod.

The Tasco No. 124 was an impressive binocular at its time of introduction in the 1960s. Its angle of field is second to none, although this construction was surely somewhat over-stretched: Stopped down to 65 degs. AFOV had still made it a wide angle, but more suited for astronomy where the edge distortions should not become too excessive. A big plus of this instrument is its compactness and low weight, achieved with its magnesium body. It is still useful for daylight applications, but its coating is of single layer and clearly behind the state of the art, making it less suitable for low light, the prime application of any 7x50 binocular.

Links:

Miyauchi's home-page
Astro Optik von Bergen, Swiss importer of Miyauchi
Fujinon Europe
The Tasco No. 124 on Fan Tao's page
Overview of classical wide angle binoculars by Fan Tao

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.

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Last updated: 2004