Binoculars review

Kowa 10x42 BD42-10

Kowa 10x42 BD42-10
30 January 2010
Kowa is a Japanese manufacturer with a very well-known, although modest, offer. Its main part consists of BD42 models, with the 42 mm objective lenses and the magnification of 8 and 10x, aimed chiefly at ornithologists. As the producer guarantees, the binoculars are designed to be the lightest and the most handy with observing a high optics quality at the same time. Their construction is based on the roof prisms in the Schmidt- Pechan system made of BaK-4 glass and phase correction coated. One of the Pechan prism’s edges doesn’t give the full reflection so it must be covered by an additional reflective layer. Kowa used a specially designed C3 coating there to minimize the light losses to only 1%. This result is by several percent better (at the extreme ends of the visible spectrum even more than 10% better) than in the case of conventionally used aluminum coatings.

The binoculars, tested here, are waterproof and nitrogen-filled.

  • Lornetka Kowa 10x42 BD42-10
  • Lornetka Kowa 10x42 BD42-10
  • Lornetka Kowa 10x42 BD42-10
  • Lornetka Kowa 10x42 BD42-10
Manufacturer data


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Magnification Lens diameter Angular field of view Prisms Eye relief Weight Price
10 42 105/1000(6o) BaK-4/roof 17.6 mm 745 g 1988 PLN
Results of the review
Real front lens diameter Left:   42.02+/- 0.05 mm
Right:  41.99+/- 0.05 mm
8 / 8.0 pkt
Real magnification 9.8+/- 0.2x 3/3.0
Transmission 86+/- 3% 13.5/25.0
Chromatic aberration In the centre on the border between medium and low; at the edge - low. 6.5/10.0
Astigmatism Slight. 7.3/10.0
Distortion The distance of the first curved line from the field centre compared to the field of vision radius : 67% +\- 7% 8/10.0
Coma Low even at the very edge. 8.5/10.0
Blurring at the edge of the FOV The blur occurs in the distance of 74% +\- 3% from the field of view centre. 4/10.0
Darkening at the edge the FOV Slight. 4/5.0
Whiteness of the image Almost perfect. 4.2/5.0
Collimation Perfect. 5/5.0
Internal reflections
Left: Right:
Kowa 10x42 BD42-10 - Internal reflections - Left Kowa 10x42 BD42-10 - Internal reflections - Right
At a low level.
Housing Very comfortable, ergonomic and stylish. Handy. Regulated eyecups. Nitorgen filled. The rubber near the objectives sticks out minimally. 7.7/8.0
Focusing Very comfortable central screw. Comfortable rubber ring on the right eyepiece. The focusing is made inside. 4.8/5.0
Tripod There is a comfortable exit. 3/3.0
Interpupilary distance from 58 to 74.1mm 5/6.0
Closest focusing distance 1.7 m. 2/2.0
Eyepieces FOV Apparent field of view of 60.4 deg (according to simple formula) and 54.4 deg (according to tangent formula). 10/20.0
Field of view Measured by us amounted to 6.01 +\- 0.03 degrees and was in perfect accordance with the specifications. 5/8.0
Quality of the interior of the barrels Dark and matt inner tube which near the prisms becomes ribbed and shiny. Clean. 4.3/5.0
Left: Right:
Kowa 10x42 BD42-10 - Vignetting - Left Kowa 10x42 BD42-10 - Vignetting - Right
Pupils slightly egg-shaped.
OL: 1.7%, OR: 2.9%.
Prisms quality High 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
15th place in the overall ranking of the 10x42 binoculars test
138.4 / 190 pkt
Econo result 12th place in the econo ranking of the 10x42 binoculars test 6.8pkt.


  • very solid and stylish casing,
  • high quality of prisms and coatings,
  • low distortion,
  • excellent coma correction,
  • slight astigmatism,
  • chromatic aberration quite well-controlled,
  • low brightness loss at the edge of the field,
  • sharp image already from 1.7 metres,
  • good blackening inside.

  • truncated pupils,
  • short guarantee period for this price class,
  • the image at the edge of the field could have been sharper.

Once again it is proven that if a given manufacturer has rich tradition when it comes to the optics production and the factories, producing its equipment, are near, it can demand a lot and have good results. In the case of Kowa it resulted in a polished-up, sophisticated product which is difficult to flaw. Very good optical properties, stylish and solid housing - those things make you keen on buying. You can carp at truncated pupils, which shouldn’t look this way in a 700 USD class device, and a short guarantee period in our country. In this class of equipment a 5-10 years of guarantee is an absolute minimum to compete successfully on the market. It is enough to look at the Olympus EXWP-I and the Vortex Viper, which cost 100 USD less and, having a score very much alike, come with a 25-year-long or even lifetime warranty. If only the tested Kowa had the same warranty period, it would improve the test score by 3-4 points and it would equal the best competitors.