Hi,
First up, good quality binos are worth buying as the optics can help a great deal and not cause eye strain/fatigue which can in turn cause head aches and long term eye damage. Nitrogen filled glass will allow you to use them in humidity without the glass clouding over and make excellent travel glasses.
Anyway, to start, to measure binos, you divide the length by the aperture which gives you your brightness factor. As such, a set of 8 x 40 = 5. As a standard thats a nice bright, good length set of bino's. Naturally, 10 x offer a slightly futher view at the cost of light so you would have a brightness factor of 4. Like a camera lens, bino's rely on light transmitting down the barrel.
Also, 10,12 and above are prone to movement, even if you have the steadiest hand in the world and in low light you will feel like you have one too many cokes. That said, the length is super to use. Anything above 10 could be tripod mounted or consider Canon IS glasses which are battery driven and use and electronic giro system to counter shake. There are two types, porro and roof prism of which roof are now pretty much the norm, leading to smaller, well performing, cost effective tools.
It is suggested that 8.5 x 32 are the ideal bino's to use as this perfectly matches the exit pupil of the human eye and is also has a very wide field of view as well as long.
I hope this helps - as mentioned earlier, Nikon Monarchs are excellent 'budget' glasses and well worth a look. Most binos work wonderfully on a nice summers day, but the acid test is low light costal work where they really come into their own.
One last thing, totally avoid some of the stuff you see in the back of the Sunday papers and on market stalls, especially the stuff with red coatings.
