Telescope Reviews: Zoom Binacular

Welcome to Cloudy Nights, davaid. You are receiving the responses of an unusual set of binocular users: some of these guys are professionals, and those who aren't are astonishingly well informed amateurs. Everything they are telling you is correct. You may not be able to interpret their more technical remarks, though, so I will attempt a layman's summary:

Zoom binoculars are very difficult to construct at an affordable price point, no matter what their magnification, and the situation gets worse the higher the magnification offered. Whether zoom or not, when binocular magnification gets much past 12X, the view is going to shake like the dickens anyway if the binocular is not set on a mount, or is not image stabilized. In addition, with a 70 millimeter binocular, there isn't going to be enough light gathering ability to illuminate a magnification much past 25X: this means that the view will be rather dim to begin with. The higher you go past 20 or 25X, the dimmer the view will get. As you go even higher, the ability of the binocular to resolve an image (bring it to a decent focus with identifiable detail) is going to break down further and further. At the same time, the size (measured in diameter millimeters) of the beam of light that exits the eyepieces (oculars) of the binocular and enters your eye will get smaller and smaller as the magnification rises (this is "exit pupil"). When the "exit pupil" gets too small (and boy will it at anything much past 40 or 50X with a 70 mm binocular), you just won't see anything at all at night. Daytime is a bit different, but the view will be grainy, at best, and will eventually be nothing at all as you approach 100X and beyond.

The references to "collimation" refer, in essence, to the ability of a two-barrelled instrument (a bi-nocular) to merge the light paths of two different optical trains into a single distinct image. Even very very expensive, high quality binoculars can run into trouble in this department at high magnifications, and it gets worse when the eyepieces (oculars) are zooms, because each eyepiece is not precisely matched to the other as you change magnifications (it is different with a "click stop" eyepiece, which locks in at a precise point, but click stop "zoom" eyepieces are very expensive and are not provided on any binocular that I know of today—though, oddly enough, I have a click stop Japanese binocular (7-12X) from 1965 that actually does merge images very well into a single image.

It is unfortunate that products like the one you are looking at are advertised and sold. It is very difficult for a beginner to know how patently unreliable the product claims are.