Binoculars allow for up-close viewing of things which are afar off. A favorite tool for sportsman and birdwatchers, but not all field glasses are created equal. A good rule of thumb to follow when planning to purchase a pair of binoculars is let how you will use them determine how to choose them.
Crunch the Numbers
The sometimes confusing numbers are the most important part of a pair of binoculars, they determine how bright and close the far away image will be to your eyes. The first number represents the power of magnification of the lens which is closet to the eye. If the fist number is 7, then an image will appear 7 times larger through the lens than it does to the naked eye.
The second number tells you how much light the 'objective' lens, or the lens farthest from your eye, will allow in. The higher the number, the larger the objective lens will be and the more light is allowed in. Larger lens allow for a brighter image, but are more difficult to carry, but are perfect for backyard bird watching. So again, it goes back to choosing binoculars for how you will use them.
Weight is something to consider when purchasing field glasses. A heavy pair is taxing on the neck and can tire the arms out quickly, but a light pair may be difficult to hold steady when trying to view wildlife. Try them out in-store for several minutes prior to purchasing them to get the best weight for your usage.
How quickly can you focus a particular pair of binoculars? The perfect magnification, brightness and weight won't matter if your quarry has disappeared by the time you get you get the binoculars focused. Practice focusing on near and far objects before making an investment.
For people who wear eyeglasses, some binoculars' field of vision is reduced by nearly half. Some manufacturers offer adjustable eye cups (sold separately) or eye relief field glasses designed for eyeglass wearers. Either adaptation will improve the field of vision when watching wildlife through a pair of binoculars.
Water-proof, rust-proof, scratch-resistant, impact-resistant and the like are all necessary durability components for field glasses that are literally going to be used out in the field. But for armchair birdwatchers, all that added expense is probably not needed. Choose binoculars according to how you will use them and you'll get many years of enjoyment from your investment.