Hutchey.
The advice above is good advice. There are the basics of exit pupil, and brightness that people usually judge binoculars by, but that will only really work with binoculars of the same quality and construction.
An example of how it doesnt work, is that I own a set of Leica geovids 8 x 42, which gives me an exit pupil of 5.25. I also own a set of cheap Bushnell 7 x 50 binos which give me an exit pupil of 7.14.
In theory the bushnells with the biggest exit pupil, should be better in low light at dawn and dusk. The reality is that there is no competion and the Leicas are absolutly s