There are several numbers. The first is the magnitifaction, the second is the final optics diameter. The larger the 1st number, the larger the 2nd number should be (more light capture) AND the harder is will be to hand-hold. My personal believe is that 10x magnification is the most anyone should do without tripod assistance (some withsteady hands can swing 12x). At 10x, I feel 50mm is optimal, at 8x I think 40mm is good. So I would recommend a 10x50 or a 8x40. This being said, for casual use $100 can definately buy you good quality. The Nikon action series are fantastic binoculars and for the price cannot be beaten IMHO. You would have to spend 2-3x more then this to get equal quality (these are much better then Bushnell costing 2x for sure).
http://www.opticsplanet.net/nikon-10...lars-7218.html
http://www.opticsplanet.net/nikon-8x...lars-7216.html
Now, if you want to see the balls on a squirrel from across the valley, you will need a scope+tripod. A lot of people will see you can get 12x50 and even 15-20x50. I really recommend against these... at those magnifications it is impossible to hold them steady. I also am against zoom binoculars (zoom scopes is another story) but they tend to be much heavier and again zoom to the 20x levels which makes hand holding hard.
More then you ever wanted to know about binoculars?
