Two important factors to consider when selecting binoculars are the power -- generally called the magnification -- and the objective lens size, which affects the clarity or detail in which you view an object. Binoculars are typically imprinted with this information, but an understanding of what the numbers mean is also important.
Check near the binocular's eyepiece for a figure expressed as a number times a second number, such as "10 x 42." The first number is the binocular power; the second is the size of the objective lens in millimeters.
Determine the focal length of the binocular tube. This isn't usually provided on the binocular body, but you should find this data in the technical information on the manufacturer's website or in the user instruction booklet.
Get the focal length of the eyepiece from the manufacturer's website.
Calculate binocular power or magnification by dividing the focal length of the binocular tube by the focal length of the eyepiece. So a binocular tube focal length of 150 mm and an eyepiece focal length of 15 mm would provide a power or magnification of 10 times normal.
Things You'll Need
focal distance of objective lens
focal distance of eyepiece
Binocular power refers to the size to which the subject being viewed is magnified versus how it appears to the unaided eye. The size of the objective lens is what determines how much light is absorbed by the binoculars and, therefore, how clear the image is. For general purposes, binoculars in the 7 to 12 power range with objective lenses of not less than 40 mm provide the best detail and magnification for most people.