Kasai Trading “Rusinov” 2.3×40

Specifications:

Objective diameter:  40 mm
Magnification:  2.3 x
Exit pupil:  —-
IPD:  59 –  74 mm
RFOV:  “28  degrees”
Focus type:  IF (direction of rotation from close to infinity: > clockwise)
Range of diopter adjustment (estimate): +/- 4.5 dpt*
Prism system:  none (Galilean)
Waterproof: no
Weight (measured, with eyepiece cover and strap): 265 g
Made in: China

Remarks:

Binoculars which do not use prisms to form an erect image but just two lenses, an objective lens with positive EFL (Effective Focal Length) and an eyepiece with negative EFL, are called Galilean, because Galileo used this type of scope for his early observations. Galilean binos do not have an exit pupil in the technical sense of the word (you could also say that they have a “virtual exit pupil”). Due to their optical characteristics, Galilean binoculars have generally low magnifications (2x to 3x) and a narrow real field of view (3 to 5 degrees), but they are usually small and light and since decades have found a use in the form of opera glasses – until Mikhail Mikhailovich Rusinov, a Russian optical designer, came along in the 1940s and designed a special Galilean type optic that provides extremely wide angles of view of 20 degrees and more with still very acceptable edge sharpness. The Rusinov type binocular has been sold for some years by various companies, the one presented here was apparently made in China and marketed by Japanese company Kasai Trading. These binos are currently sold as “starfield binoculars” since they are a usefool tool for panning the night sky; thanks to their wide field of view, entire constellations can be viewed at once.

Other, similar devices are sold by Vixen, Omegon, Astro Hutech and Orion (see separate posts “Vixen SG 2.1×42” https://binocular.ch/vixen-sg-2-1×42/, „Omegon 2.1×42“ https://binocular.ch/omegon-2-1×42/, “Orion 2×54” https://binocular.ch/orion-2×54-ultra-wide-angle/ and „Astro Hutech Hinode 1.8×40“ https://binocular.ch/astro-hutech-hinode-1-8×40/).

see also separate post “Kasai Trading 3×50”, https://binocular.ch/kasai-trading-3×50/

Ratings:

Review:

A brief 2014 review (in German) can be found here:
https://www.juelich-bonn.com/jForum/read.php?9,422922,423004#msg-423004

A comparison with the Vixen SG 2.1 x 42 here:
https://www.juelich-bonn.com/jForum/read.php?9,428946,428946#msg-428946