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8X25 binoculars are pretty useless for astronomy, they're better than nothing - good for constellations - but you won't see the brighter deep sky objects very well.
7X50 binoculars are pretty good at dark sites and excel for looking at the Milky Way star fields, but you will struggle to spot galaxies with them as easily as with 10X50s.
10x50s show you more galaxies, and are better in suburban backyards than 7X50s, as the background light in the image isn't as bright.
8x42s and 10X42s as sold at Andrews are pretty good too, especially if you have astigmatism as they have smaller exit pupils than 7 and 10X50s do, but you won't see quite as deeply with them as with the 50mm lens binoculars.
15X50s are actually very good on deep sky objects too, but they don't weigh much and thus shake a lot when held hand. I have my Pentax pair on a tripod, and hold onto the tripod when viewing, as the extra weight steadies them.
10X70s are superb for looking at Milky Way star fields at a dark site.
After one gets experienced with the smaller binoculars and can find things in the sky, bigger pairs with smaller field of views, but more magnified and brighter images that show more objects become very attractive.
I use 15X70s and 18X80s that I hand hold without much problem (but some people do have a problem), but I occasionally make them steadier by attaching them to a tripod and holding the tripod.
20x60s and 20X80s need a tripod, and usually to have it on the ground and sitting behind it - or one of those binocular counterweight gadgets on a tripod for more comfortable views.
25X100 are fabulous at a dark site. Sweep through Virgo and dozens of galaxies pop out without any difficulty. But a heavy duty tripod is essential, preferably with a handle that one uses to wind the binoculars up and down the central shaft.
The difference between expensive and cheap binoculars of the same size is the quality of the image. Cheap binoculars aren't as sharp in the middle, of the image, and stars distort towards the edges. While expensive ones tend to be razor sharp across the field.
Also, more expensive binoculars have better contrast - you see it especially during daytime, the images in them just look more real than in cheaper ones. The best pair of binoculars I've ever looked through were Leica 10x42, but they cost $2500 and I didn't buy them. The contrast in them was noticeably better than in the cheap $400 and $800 pairs I compared them too.
My Nikon 10X70s and Fujinon 7X50s are the only ones of mine I'd rate as being in the premium league, but I'm not that fussy - I happily used a a K-mart pair of 10X50 binoculars that cost me $35 to see scores or deep sky objects, before realizing that there were much better pairs around.
Also you can get Image Stabilised binoculars, which are magical - you press a button and the shaking stars just stop shaking. I have a pair of Canon 16X50s, and they give great views of the moon and deep sky objects, but strangely enough, I haven't abandoned my other binoculars.
Regards,
Renato
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