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Types of binoculars and terms you should know
When shopping for binoculars, the magnification and aperture numbers tell you how much detail you'll be able to see and roughly how large the binoculars will be. For binoculars described as "8x42," for example, the "8" is the magnification (or power). This means the binoculars make an object look eight times closer than through the naked eye. The number "42" means that the lens furthest from your eyes -- called the objective lens (also called aperture or front end) -- is 42mm. The size of the objective lens determines the bulkiness of the binoculars. A larger objective lens lets in more light, so binoculars with larger objective lenses are theoretically better in low light.
The price range you choose usually determines the type of binocular you buy. Expensive, top-quality binoculars are usually H-shaped, roof-prism binoculars -- which are compact and usually dustproof, waterproof and fogproof. (Fogproofing makes the binoculars usable even in very cold weather, when most binoculars would fog internally.) Less expensive models are usually W-shaped porro-prism binoculars (not waterproof or fogproof), or reverse porro-prism compact binoculars.
Compact binoculars (which are handy for concerts, hiking or general use) have objective lenses 30mm or smaller. They're lightweight and convenient, but usually provide less detail and brightness. Mid-size binoculars, with objective lenses of 32mm to 35mm, are often more comfortable to hold than full-size binoculars, but offer a dimmer view. Full-size binoculars, with objective lenses of about 42mm to 50mm, usually offer the brightest image. You'll see some giant binoculars that are even larger. These are often used for star-gazing (though 10x50 is also a good size for astronomy binoculars) and are best used with a tripod.
For general use, experts usually recommend 8x42 binoculars, especially if you don't want to pay more than about $500. You can see more detail with 10X magnification, but unfortunately, any flaws in the optics or design of the binoculars are also magnified. For hunting binoculars or birding binoculars used across big open fields, or for watching ocean birds, it could be worth buying top-quality 10x50 binoculars that cost more. If you want to watch close objects, such as birds at feeders or butterflies, you'll want binoculars that can focus especially close -- a growing trend in binocular technology. For beginning birdwatchers, experts recommend 7x35 binoculars because their wide field of view makes it easier to get the targets in view -- something that takes practice. "Field of view" is a measurement (in feet) of the diameter of the image you'll see 1,000 feet away. A wider field of view means you'll see more of a landscape or sports field without shifting your gaze.
Eye relief (in millimeters) is another specification you'll find in descriptions of binoculars. It's simply the optimal distance between your eye and the near lens on the binoculars (called the ocular lens). Reviews recommend a minimum of 14mm eye relief for people who wear glasses, to avoid "tunnel vision" -- a reduced image surrounded by black. But eye relief is important for overall eye comfort for all users. Reviewing binoculars at BetterViewDesired.com, Wayne Mones explains that "The closer your eye has to be to the eyepiece the more fatigue the glass will cause. Having your eye close to anything continually triggers the blink reflex." The best binoculars have adjustable eye relief, with twist-up eyecups that offer several settings from which to choose.
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