Binoculars are used for a multitude of outdoor & indoor activities. From sporting events, to bird-watching, no matter what activity you use them for, they will always be used to improve your view of distant objects.
How you plan to use your binoculars, and when and where, are critical to the selection process. It's important to evaluate binocular features so you may determine which are important to you.
Every pair of binoculars has a reference number that tells you its magnification power. An example would be 8 x 42. The number "8" refers to how many times the binoculars will magnify an object; in this case, it will appear 8 times closer than it actually is. As a general rule, higher magnification will make it more difficult to pick up moving objects.
The number "42" of 8 x 42 indicates the diameter of the front of the binoculars, or the "objective lens". The objective lens gathers the light that will eventually reach your eyes. In general, a larger objective lens means more light will be let in the viewing area, also known as the field of view. A larger lens diameter, however, does mean a heavier pair of binoculars.
Binoculars come with a variety of brightness levels; this is essentially the amount of light your binoculars let in. The primary deciding factor in determining brightness is the size of the exit pupil, which is the size of the beam of light your binoculars allow in. A larger exit pupil will allow light to enter, which is a benefit if you plan to use your binoculars in lower light conditions but not desirable if they will be used outdoors in the sunshine. To determine the size of the exit pupil, divide the lens diameter by the magnification power.
Field of View:
This is the measure of how much you can see through your binoculars at 1000 yards. Generally, the higher the magnification, the less the field of view.
The purpose of a prism in binoculars is to correct the inverted and reversed images you would see in their absence. The prisms are located inside the binoculars and they transmit light from the objective lens to the eyepiece. Binoculars come in prism designs: "porro" prism and "roof" prism. A "porro" prism offsets the eyepieces from the objective lens to allow more brightness to be let in. However, this makes the binoculars larger. A "roof" prism aligns the lenses in a straight configuration, which makes the design smaller. This preferred size is a consideration, but the image is not as bright as with a "porro" prism.
There are generally three focus features on each pair of binoculars. Some binoculars offer fixed focus, which is convenient but loses the ability to alter the focus for personal needs. A center knob lets you focus both barrels at the same time. The right diopter ring, located on the right eyepiece, allows you to customize your focus for each eye. Binoculars are also ranked on near-focus distance, which is simply how far away you must be from your subject before it can be properly focused. The general range is 10-40 feet.