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There are some key points that are used to rate binoculars. Size is one aspect, with binoculars coming in full-size, mid-size, and compact versions. The fit for each person will vary and those that fit a person better will be ideal and rated higher by that person. In the size also come the performance differences among binoculars. Full-size binoculars capture light better and are better for gaining a wide view. This makes them the best in terms of performance. As the size decreases, so does the performance. This is not to say that smaller is worse, simply that larger binocular have greater ability. 

Each pair of binoculars is rated with two numbers. The first number will tell you the magnification power. The second number is the diameter in millimeters of the larger lens of the binoculars. http://www.ask.com/question/how-to-rate-binoculars