There is much to consider when considering the purchase of a pair of binoculars. The most desired end result is a pair that will provide the magnification needed, the brightness and clarity of the image desired, and the tolerable weight of the unit being carried around a full day all coupled with a reasonable price. But getting there is really not that simple. There are trade offs that have to be understood and considered; and those trade offs are then weighed against your expectations.
For example lets start with magnification. The larger the magnification the smaller the field of view and the heavier the binoculars will be. While at first glance a larger magnification might be desired, most people find that a magnification of 12x and higher are too difficult to hold steady to get a satisfying view. At higher magnifications a tripod is strongly recommended. That also raises the question of how much stuff do you want to drag along. Most popular magnifications are in the 7 to 10 power.
Brightness of the view is next to consider, however we have to make sure that usage times are taken into account. If you need to utilize the binoculars during pre-dawn or at dusk, then larger objective lenses are certainly recommended. Generally speaking, larger lenses do provide a brighter image than smaller lenses but will also mean a heavier weight to carry. To clarify further, the bigger is better theory only applies if all optical elements are equal. This is because different coatings and glass quality can drastically affect light transmission. For example, an 8x42mm model can provide a brighter and sharper image than an 8x50mm model that happens to use lower grade glass for the prisms without multi-coatings. In those cases you are certainly not paying for the name.
There are other things that must also be considered such as the close focus specification, eye relief, exit pupil, and prism styles. To learn much more about binocular specifications click on over to our binocular selection guide or Optics University.