Binoculars are simple instruments with only one purpose: to make distant objects appear closer. There are five key specifications, that must be considered, before buying them. The first two are Power and Objective Diameter. They always go hand-in-hand. The next two are Field of View and Eye Relief. The last is Minimum Focusing Distance.
Power and Objective Diameter always go along with an "X" between them. The power number shows how many times closer the subject will appear. The number after the "X," is the objective diameter, describing the amount of light that can pass through the binoculars at a given power. The actual measured light transmission is affected by things like anti-reflection lens coatings, glass composition, prism material, etc.
The next specification is Field of View at 1,000 Yards. The higher the number, it shows that the binoculars are wide angle. At a given power one can see more from edge-to-edge, and that is beneficial at ball games and auto races, but not as critical for birders.
Eye Relief is used to show the distance between the binoculars and the eye and still can be used comfortably. For those who wear eyeglasses, look for eye relief numbers in the 18mm to 21mm range. Some can be used with eyeglasses and some must be folded out of the way.
Minimum Focusing Distance. Some binoculars focus on objects very close while others must be far away. A law enforcement officer surveilling an airport will go for glasses that focus on nearby subjects, while Baseball fans in the cheap seats will not notice any difference.
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