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Comet 6x24 binoculars from the inside – what went wrong?

28 September 2017
Marcin Górko

3. Focusing mechanism

The focusing mechanism is the simplest construction possible – individual focusing is done through a helicoid which work is smoother after my intervention, with better resistance across the range. Nevertheless at first the binoculars performed strangely: after adjusting the focus of both eyepieces on the same object with the same eye the left eyepiece was more extended by 1.4 mm. I thought it was because of the wrong assembly of focusing rings (see my remarks above). Still, after removing these rings I tried to focus with just the tubes of the eyepieces and the problem remained. As both eyepieces and their tubes are identical there are two possible causes: either one of the eyepieces is more shallowly set in the tube than the other or one of optical paths is by 1.4 mm longer than the other due to a possibly faulty assemblage of the prisms. Gaps in the focusing mechanism, described by Arek in his test, are also a consequence of too loose joint between the tube of the eyepiece and the focusing ring. When the resistance on the helicoid reaches a certain value it is easier to turn the focusing ring on the tube of the eyepiece than the tube inside the helicoid. Each of these gaps of course made the discrepancy between the dioptre scale and the real setting more and more pronounced. Still one thing is sure: the producer didn’t stint on grease in the helicoid. The problem is that too much grease creates too big focusing resistance; apart from that, the excess of grease can make the inner part of the prism or the element of the eyepiece dirty... believe me, you wouldn’t like to remove grease from an optical surface covered by MC coatings.

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Comet 6x24 binoculars from the inside – what went wrong? - Focusing mechanism
Left: a tube with optics of the eyepiece (before the blackening), a slide and a hole in the prism housing where that slide is screwed in. Copious amounts of viscous grease are clearly visible; there are also a lot of debris on the thread in the prism housing which is supported by the second eyepiece.

The slide in the prism housing is attached in the only sensible way – by a fine thread. It must be said that the thread is very neatly cut, both on the slide and in the prism housing. Unfortunately after unscrewing the eyepiece you see a lot of small pieces of debris inside the binoculars, glued by the grease to the aluminum cast. Not cleaning the interior of the binoculars before the final assembly is a serious negligence of the producer. These flecks, probably hard aluminum shavings, can be in time transferred on the surface of optical elements. They also might fall on the thread of helicoid, making the focusing far less comfortable.