The 8x40 BM2-0840 (United-Optics)

by Holger Merlitz

This binocular is manufactured by one of Kunming's numerous optical plants and exported by United-Optics (see link below). It exists in various versions, with individual or central focuser, with thick rubber armor or just a rubber skin, with- and without military reticle. One of its variants seems to be the Oberwerk Mariner (though I have no confirmation on that), which is sold for around 150 US Dollars.

  Real angle Apparent angle Eye relief Exit pupil Weight
  of view (deg) of view (deg) (mm) diam. (mm) (kg)
BM2-0840 8.3 66 19 5 1.0 (a)

a: approximate

The present sample is the military version with reticle (right hand ocular) and rubber armor. The mechanical construction appears very robust, all moving parts are going precisely and without play. This unit is claimed to be nitrogen purged and water proof, and efforts to achieve that are visible: The prism-house front plates are fixed with no less than 5 screws to keep the construction water tight. The objective covers are made of rubber and firmly snap-in to protect the lenses against dust and rain. If removed, they fully detach from the body and have to be stored separately. The oculars are covered with a rain guard, and their eye-cups are made of soft rubber that folds either up or down (for use with eye-glasses), intermediate positions are not possible. The eye-relief is generous, there is a quoted value of 19mm. The close focus seems to be around 5m. The general construction of this device appears to be of quality, comparable with the Romanian IOR-SA 7x40. The weight is with roughly 1 kg quite high for a 40mm binocular, although not unusual for a military unit.

The image shows a wide field of view of 66 degs. apparent angle. The central 60% of the field (radial) display stars that are point like, beyond that aberrations gradually increase. The outermost 10% exhibit a very high level of blur. For daylight applications about 80% of the field appear reasonably sharp. Bright light sources in the night produce some ghost images, but their intensity remains low-to-moderate. Similarly, the full moon is showing some reflexes of only low intensity. Compared to several Chinese binoculars I have seen earlier, the quality of coating appears somewhat improved. I have seen the intensity of ghosting being a bit higher through the left barrel than through the right (as a surprise, since the reticle is placed in right barrel). This may indicate a quality scattering of the anti-reflection coating. This binocular produces a moderate stray light under difficult light conditions (twilight, when observing a shadowed forest under bright skies), but not otherwise (during daylight or night observations). The color rendition is neutral, and the image appears bright throughout, without any visible trace of vignetting. With its short and wide body shape, its objectives are placed rather far apart, creating a nice stereoscopic performance of this binocular.

In summary, this is a very rugged binocular with a reasonable optical performance and a fair price tag. Mechanically, it feels as robust as the IOR-SA. Optically, the Romanian glass shows less stray light and ghosting, indicating the compromise one has to take with the cheaper Chinese glass. In 90% of practical applications, these differences remain undetectable, but in critical light conditions the IOR can play out its extra performance margin. The BM2-0840 is perhaps not particularly suited for astronomy, because the edge sharpness is not good enough. Bird watchers will complain about the close focus of 5m. For traveling, hunting, sailing and any kind of rugged outdoor activities this is a fine glass. There exist large versions of this binocular with up to 100mm objectives (e.g. the Oberwerk 25x100 IF) which are said to be excellent performers in astronomy.


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 purchased this sample for testing and payed the factory price.



Oberwerk Mariner series

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Last updated: Sept. 2005