The question of zoom binoculars is one that inevitably arises, not least because there are good quality zoom telescopes and good quality astronomical zoom eyepieces on the market. I once made the comment that a decent zoom binocular for astronomy had yet to be invented. A vastly experienced binocular repairman, Bill Cook, retorted to the effect that my qualification "for astronomy" was redundant. The reasons for this are simple. Not only must the eyepieces zoom to within 1 percent of exactly the same rate (which means absolutely no perceptible rocking of the bridge), but a zoom binocular requires a system with moveable optical elements that must hold collimation, ideally to better than an arcminute where step (a.k.a. dipvergence, a.k.a. supravergence) is concerned if one is approaching x30; for the x125 that I have seen advertised for some zoom binoculars, this translates to better than 15 arcseconds! Now, consider how many good quality center-focus x30 binoculars you know of—I don't know of any, and I am sure that part of the reason must be that it would be a feat of technological brilliance (not to say expense!) to bridge two eyepieces in such a way that they maintain collimation to within the tolerances that are required. (And remember that it is unlikely that they will have a "base tolerance" of zero error.)
According to Seyfried, zoom binoculars were developed as a "gimmick to stimulate sales" on the back of the success of zoom lenses for cameras.5 He also asserts that the frequency with which they fail results in their being disproportionately represented at binocular repair facilities and states that he has never seen a zoom binocular that can hold collimation.
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