Binoculars review

Kowa 8x42 BD42-8

Kowa 8x42 BD42-8
10 October 2012 
Kowa is a reputable Japanese optics producer. Not so long ago all their products were made in Japan and the models from Genesis and BD series enjoyed an especially good opinion. Lately that company decided to enlarge its offer, including a bottom-of-the-range price segment, with binoculars produced in China.

The 8x42 BD-42-8 model, tested here by us, belongs to a group of binoculars produced still in Japan. That series consists of devices with the following parameters: 8x25, 10x25, 8x32, 10x32, 8x42 and 10x42. As the company claims, these binoculars were designed in such a way so they are physically the lightest and the most handy, while their high-quality optics remains intact. Their construction is based on Schmidt-Pechan roof prisms, made of BaK-4 glass, with phase correction coatings. One edge of the Pechan prism doesn’t ensure you a complete reflection so it must be covered by an additional reflection layer. Kowa used a highly reflective C3 coating which reduces the losses of light to just 1%. That result is by several percent better (at both ends of the visible spectrum even by over 10% better) than in the case of standard aluminum coatings.

The binoculars are waterproof, nitrogen-filled and they come with a two-year warranty period.

  • Lornetka Kowa 8x42 BD42-8
  • Lornetka Kowa 8x42 BD42-8
  • Lornetka Kowa 8x42 BD42-8
  • Lornetka Kowa 8x42 BD42-8
Manufacturer data


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Magnification Lens diameter Angular field of view Prisms Eye relief Weight Price
8 42 110/1000(6.3o) BaK-4/roof 18.3 mm 730 g 2197 PLN
Results of the review
Real front lens diameter Left:   42.02+/- 0.05 mm
Right:  42.07+/- 0.05 mm
8 / 8.0 pkt
Real magnification 8.05+/- 0.05x 3/3.0
Transmission 84+/- 1.5% 11/15.0
Chromatic aberration Low in the centre, a bit higher than medium on the edge. 5.5/10.0
Astigmatism Very low. 8.5/10.0
Distortion Distance between the first curved line and the centre of the field compared to the field of view radius: 33% +\- 4% 3/10.0
Coma Starts near ¾ of the field of vision and is medium on the very edge. 7.4/10.0
Blurring at the edge of the FOV The blur occurs in the distance of 86% +\- 3% from the field of vision centre. 7/10.0
Darkening at the edge the FOV Slight. 4/5.0
Whiteness of the image Significant difference between the transmission for red and green-blue light. You shouldn’t observe something like this in that price segment. 3/5.0
Collimation Perfect. 5/5.0
Internal reflections
Left: Right:
Kowa 8x42 BD42-8 - Internal reflections - Left Kowa 8x42 BD42-8 - Internal reflections - Right
Quite decent. No significant light areas.
Housing Very comfortable, ergonomic, stylish and handy. High quality rubber padding which sticks to your hands very well. Rubberized, regulated eyecups with as many as 5 stops. The rubber near objectives comes off a bit. Made in Japan. 7.7/8.0
Focusing Comfortable, ribbed central wheel. It works smoothly in a range of 450 degrees. Comfortable, ribbed ring, made of rubber on the right eyepiece for individual focusing. It moves the outer element. 4.5/5.0
Tripod There is a comfortable exit. 3/3.0
Interpupilary distance from 58.3 to 74.1mm 5/6.0
Closest focusing distance 1.95 metres. 2/2.0
Eyepieces FOV Apparent field of view of 51.5 degrees. 5/10.0
Field of view Measured by us amounted to 6.4 +/- 0.04 degrees and it was a bit woder than stated in specifications. A narrow field of view for this class of equipment. 5/8.0
Quality of the interior of the barrels Dark and mat inner tube near the prisms becomes ribbed and shiny. Clean inside. 4.3/5.0
Left: Right:
Kowa 8x42 BD42-8 - Vignetting - Left Kowa 8x42 BD42-8 - Vignetting - Right
OL: 2.7%, OR: 10.8% The right exit pupil truncated very significantly.
Prisms quality Good quality BaK-4. 8/8.0
Antireflection coatings Greenish on objectives, prisms and eyepieces. Medium intensity. 4.5/5.0
Warranty [years] 2 2/6.0
Final result
121.5 / 170 pkt
Econo result 0pkt.


  • solidly made, stylish casing,
  • low astigmatism,
  • sensibly corrected coma,
  • slight light fall-off on the edge of the field of view,
  • good quality prisms made of BaK-4 glass,
  • clean and dark inside the tubes.

  • narrow field of view,
  • noticeably truncated exit pupils,
  • weak distortion correction,
  • noticeable chromatic aberration on the edge of the field of view.

Although our test result of the Kowa 8x42 is good, the overtone of our summary won’t be positive. That Japanese company decided to use a narrow field of view. We can understand their reasons but not without difficulty. If the field is so narrow you should get something else instead. For example you can expect the field which is corrected perfectly well till the very edge. Here there is no such thing, though. Quite the opposite in fact.

The distortion is corrected weakly, the chromatic aberration on the same edge of the field as well. You see neither good, high resolution, kept to the very end of the field, nor a perfect coma correction. Additionally there are significantly truncated exit pupils and an average transmission function graph (presented below). Faults like these we could accept in the case a roof prism instrument for ~150 Euro but from a Japanese piece of equipment costing almost 500 Euro we expect much more – certainly not a transmission level of 82-85% in the centre of the visible spectrum.

The Kowa BD 10x42 model fared very well in our tests. The field of view of eyepieces was sufficiently wide so the pair of binoculars could boast of a significant field, reaching 6 degrees. What’s interesting, that field was corrected as well as in the 8x42 model. We encouraged the purchase of the Kowa 10x42 but if you want to buy the Kowa 8x42 we advise you to think that decision over seriously. You can find several better options at such a price.

Pentax 8x43 ED DCF and Kowa 8x42 BD42-8