Because binoculars enjoy many uses, it's important to make sure that the type you select is properly designed for your purposes. Bird watching binoculars, hunting binoculars, marine binoculars, spectator binoculars, or travel binoculars all have design specifications that make them uniquely suited for their particular function. Of course, the two major concerns when it comes to choosing any compact binoculars are magnification and weight. Some pairs, like those used for astronomy, are so large and heavy that they require a binocular tripod for support. These would be ill-suited for activities such as bird watching or fast-action sporting events.
By the numbers
The different types of binoculars are distinguished by a numerical designation, i.e. 8x40. The first number represents the power of magnification. In the above example, the 8 means that the image appears eight times closer than it is. The second number 40 is the diameter of the front lens measured in millimeters. The larger the diameter of the front lens, or objective, the more light that can enter, and the clearer and brighter the image. Another number which is important is the field of view. A field of view of 250 feet at 1,000 yards means that if you viewed a train from 1,000 yards away, your view would include a section of train 250 feet long. In general, the greater the magnification, the less the field of view is. Some binoculars, however, come equipped with wide-angle eyepieces that increase field of view without decreasing magnification.
The following gives general guidelines for selecting binoculars: