Hi
On the subject of 10x50 versus 10x25..
Yep, the magnification is the same, the 50mm objective lenses let in 4 times as much light as the 25mm lenses (you square the diameter in mm to compare light-gathering ability)
25mm lightweights are good for daytime use, but in low light - or for astronomy purposes, 50mm binocs will be streets ahead (assuming similar quality)
On the subject of zooms..
A few major reasons why they're bad for astronomy.
First off - the field of view in zooms is much narrower than in fixed magnification binocs - making it much harder to navigate your way round - and restricting your wide views of Milky Way starfields and large Deep sky objects like the Andromeda galaxy.
Zoom mechanisms require many more pieces of glass, than fixed magnification - and every piece of glass that the light goes through, diminishes the brightness of the image (all-important for astronomy) - for example, a 60mm zoom will probaly only give images as bright as the average 50mm fixed mag binocs.
Zoom mechanisms are also VERY difficult to build accurately aligned - and as the power goes up - the finer the tolerances required become - as a result, the alignment of the two images is usually only ever *good* at low powers - as you zoom, the alignment wanders.
In daylight, this isn't so much of a problem - your eyes and brain can merge slightly mis-aligned images, if the have sufficient light to work with.
At night, you can never merge the bad mis-alignment that you get at high powers in zooms, and the slight mis-alignment that you get at low-powers, will give you a headache within 10 or 15 minutes of viewing.
To manufacture an ACCURATELY aligned high power zoom binoc, would cost so much, that they would only appeal to a specialist market - and a low production run would just shove the price up even further - so none of the manufacturers bother, and all we see are cheapies, which are frankly awful.
I actually own one pair of zoom binocs (10-30 x 60mm) which I keep for the sole purpose of reminding me never to buy another zoom binoc - If I'm ever tempted - one look through those, and I come bac to reality.
