How to Calculate Binocular Magnification

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.

Close Up of Businessman Using Binoculars, Reflection
Binoculars can be a fun way to use your time.
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Step

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.

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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.

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Get the focal length of the eyepiece from the manufacturer's website.

Step

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.